Years ago, I heard Anthony Burgess speak at the Edinburgh book festival. He was impressive in that he spoke for an hour without a single note, and was fluent and coherent. But of the content of his talk all I remember are his opening words: "For me, death is already sounding its high C." This was around 1980, I think, so he was in his early 60s at the time, and died in 1993. I was in my late 40s, and he seemed to me – not old, exactly, but getting on a bit.

Today, people in their 60s seem – not young, just nicely mature. Old age is in the eye of the beholder. I am 80, so I am old, no question. The high C is audible, I suppose, but I don't pay it much attention. I don't think much about death. I am not exactly afraid of it, though after reading, with admiration, Julian Barnes's book Nothing to Be Frightened of, I felt that I had not sufficiently explored my own position on the matter. But perhaps I have arrived at the state of death-consciousness that he identifies – we cannot truly savour life without a regular awareness of extinction. Yes, I recognise that, along with the natural human taste for a conclusion: there has been a beginning, which proposes an end. I am afraid of the run-up to death, because I have had to watch it. But I think that many of us who are on the last lap are too busy with the baggage of old age to waste much time anticipating the finishing line. We have to get used to being the person we are, the person we have always been, but encumbered now with various indignities and disabilities, shoved as it were into some new incarnation. We feel much the same, but clearly are not. We have entered an unexpected dimension; dealing with this is the new challenge.

Old age is the new demographic, and you can't ignore the problems created by a group that has been getting steadily larger – alarmingly larger if you are in the business of allocating national expenditure. The poor have always been with us, and now the old are too.

We have not been, in the past, and we are not so much around still in some parts of the developing world. But in the west we are entrenched, bolstered by our pensions, brandishing our Freedom Passes, cluttering up the surgeries, with an average life expectancy of around 80. But our experience is one unknown to most of humanity, over time. We are the pioneers, as an established social group gobbling up benefits and giving grief to government agencies. Before the 16th century, few people saw 50, let alone 80. Scroll back, and average life expectancy diminishes century by century; 2,000 years ago, it stood at around 25. That said, the old have always been around – it seems that perhaps 8% of the population of medieval England was over 60 – but not as a significant demographic group, rather as noticeable individuals.

The Bible blithely allowed for threescore years and 10 – where on earth did they get that from? You'd be lucky indeed to make that in the Middle East in Roman times. Life expectancy is of course a slippery concept. The trick is to get through infancy, then the next four years; notch those up, and you're in with a fighting chance – if you are a medieval peasant (or in much of sub-Saharan Africa, or Afghanistan, today) you may well hang in there till 40 or beyond. But chances are you might not leave toddlerhood; the underworld is a teeming sea of tiny ghosts, with, dotted among them, out of scale, inappropriate and incongruous, the exhausted figures of the old. Think Sparta (babies exposed on hillsides), think Coram's Fields (London hospital for foundlings), think Hogarth, think Dickens. Think Kindertotenlieder.

A recent survey by the Department for Work and Pensions, which is somewhat obsessed with the question of old age, for good reason, found that most believe that old age starts at 59 while youth ends at 41. People over 80, on the other hand, believe 68 to herald old age, while 52 is the end of youth. Of course, of course – it depends where you happen to be standing yourself. And youth has expanded handsomely since Charlotte Brontë wailed, "I am now 32. Youth is gone – gone, – and will never come back; can't help it." It still won't come back, even after a century and a half of scientific advance, but there is plenty of remedial work on offer by way of nipping and tucking for those feeling a bit desperate. The rest of us settle for the inevitable sag and wrinkle, and simply adjust our concept of the climactic points. Actually, I'd step out of line and go for 70 rather than 68 as the brink of old age; I have too many vigorous and active friends in their late 60s and anyway the round number is neater.

By 2030 there will be 4 million people over 80 in the United Kingdom – out of a population of around 60 million. No wonder the Department for Work and Pensions is getting rattled on behalf of its successors. I will have handed in my dinner pail and my Freedom Pass by then, I sincerely hope, though I can't quite count on it. I come from a horribly long-lived family. My mother died at 93; her brother made it to 100; their mother reached 97. I look grimly at these figures; I do not wish to compete.

You aren't going to get old, of course, when you are young. We won't ever be old, partly because we can't imagine what it is like to be old, but also because we don't want to, and – crucially – are not particularly interested. When I was a teenager, I spent much time with my Somerset grandmother, then around 70. She was a brisk and applied grandmother who was acting effectively as a mother-substitute; I was devoted to her, but I don't remember ever considering what it could be like to be her. She simply was; unchangeable, unchanging, in her tweed skirt, her blouse, her Shetland cardigan, her suit for Sunday church, worn with chenille turban, her felt hat for shopping in Minehead. Her opinions that had been honed in the early part of the century; her horror of colours that "clashed"; her love of Tchaikovsky, Beethoven, Berlioz. I never thought about how it must be to be her; equally, I couldn't imagine her other than she was, as though she had sprung thus into life, had never been young.

Old age is forever stereotyped. Years ago, I was a judge for a national children's writing competition. They had been asked to write about "grandparents"; in every offering the grandparent was a figure with stick and hearing aid, knitting by the fireside or pottering in the garden. The average grandparent would then have been around 60, and probably still at work. When booking a rail ticket by phone recently, I found myself shifted from the automated voice to a real person when I had said I had a Senior Railcard, presumably on the grounds that I might get muddled and require help – which was kindly, I suppose, but I was managing quite well. We are too keen to bundle everyone by category; as a child, I used to be maddened by the assumption that I would get along famously with someone just because we were both eight.

The stereotypes of old age run from the smiling old dear to the grumbling curmudgeon. In fiction, they are rife – indeed fiction is perhaps mainly responsible for the standard perception of the old, with just a few writers able to raise the game. Muriel Spark's Memento Mori is a black comedy, with a group of elderly people plagued by sinister phone calls: "Remember you must die." No stereotypes, but a bunch of sharply drawn individuals, convincingly old, bedevilled by specific ailments, and mainly concerned with revisions of their pasts in terms of will-making and the machinations of relationships. Kingsley Amis also went for comedy, in Ending Up, with a group cohabiting in a cottage and busy scoring points off each other – funny, but with a bleak undertone. Saul Bellow's Ravelstein is neither comedy (though not without humour) nor stereotype, but strong writing about the view both of and from old age. And he was old – 84 – when the book was published, whereas neither Spark nor Amis were – Spark was 41 when Memento Mori came out. Just three examples; they spring to mind simply because memorable and effective writing about old age is rare, though there are of course other instances. Old age seems to be a danger zone for many novelists, somehow even more of a challenge than the universal problem of writing about and from the point of view of a man if you are a woman, and vice versa; we all have to deal with that unless we are to be left with a very curiously populated novel. But the old and the young are, somehow, the elusive element; equally, few novelists are good at children.

"What do they think has happened, the old fools, / To make them like this?" Any reference to Philip Larkin's poem in this context is almost a cliche. The poem marries perception of age with stark truth: "Well, / We shall find out." He never did, of course, dying at 63. And the perception is of drooling, confused, incapable old age – not a stereotype so much as an evocation, both harsh and reflective.

Am I envious of the young? Would I want to be young again? On the first count – not really, which surprises me. On the second – certainly not, if it meant a repeat performance. I would like to have back vigour and robust health, but that is not exactly envy. And, having known youth, I'm well aware that it has its own traumas, that it is no Elysian progress, that it can be a time of distress and disappointment, that it is exuberant and exciting, but it is no picnic. I don't particularly want to go back there.

‘This old-age self is just a top dressing’ … Penelope Lively. Photograph: Eamonn Mccabe

And in any case, I am someone else now. There are things I no longer want, things I no longer do, things that are now important. Writing survives, for me – so far, so far. Other pleasures – needs – do not. I was a gardener. Well, I am a gardener, but a sadly reduced one, in every sense. I have a small paved rectangle of London garden, full of pots, with a cherished 20-year-old corokia, and two pittosporums, and various fuchsias, and Convolvulus cneorum and hakonechloa grass and euphorbia and heuchera and a Hydrangea petiolaris all over the back wall. It gives me much pleasure, but is a far cry from what I once gardened – a half acre or so that included a serious vegetable garden. All I can do now is potter with the hose in summer, and do a bit of snipping here and there, thanks to the arthritis; forget travel, what I really do miss is intensive gardening. Digging, raking, hoeing. Pruning a shaggy rose: shaping for future splendour. Dividing fat clumps of snowdrops: out of many shall come more still. And that was – is – the miraculous power of gardening: it evokes tomorrow, it is eternally forward-looking, it invites plans and ambitions, creativity, expectation. Next year I will try celeriac. And that new pale blue sweet pea. Would Iris stylosa do just here? Gardening defies time; you labour today in the interests of tomorrow; you think in seasons to come, cutting down the border this autumn but with next spring in your mind's eye.

Can't garden. Don't want to travel. But can read, must read. For me, reading is the essential palliative, the daily fix. Old reading, revisiting, but new reading too, lots of it, reading in all directions, plenty of fiction, history and archaeology always, reading to satisfy perennial tastes, reading sideways too – try her, try him, try that.

This someone else, this alter ego who has arrived, is less adventurous, more risk-averse, costive with her time. Well – there is the matter of the spirit and the flesh, and that is the crux of it: the spirit is still game for experience, anything on offer, but the body most definitely is not, and unfortunately calls the shots. My mind seems to be holding out – so far, so far. My poor father had Alzheimer's; that shadow lies over all of my age group, with the number of sufferers now rising all the time – but that is of course a factor of the new demographic.

Professor Tom Kirkwood has written: "There is a little progress with age-related diseases." But he went on to say that in his study of a group over 85 not one had zero age-related disease, and most had four or five. Doctors' surgeries and hospital waiting rooms are well stocked with those over 65; it is the old and the young who demand most attention. Over the last years, I have had surgery and treatment for breast cancer; hips and knees are holding out so far but my back gave in long ago: I have been in intermittent pain for 15 years – discomfort always, tipping into real pain. My sight is dodgy – myopic macular degeneration, which may get worse (but also – fingers crossed – may not). There is a shoulder problem – a torn tendon. The worst was a cracked vertebra, four years ago, which required surgery – balloon kyphoplasty – which left me in intense, unrelenting and apparently inexplicable pain for three and a half months. Pain that had the specialists shaking their heads, baffled, passing me around like the unwelcome parcel in that children's game – and I am sorry, apologetic, through the miasma of pain, sorry to be such a challenge, but sorrier still for myself.

I have sometimes wondered if an experience like that has some salutary value for any of us: it puts into perspective subsequent distresses. As for the rest of my continuing ailments, they seem more or less par for the course for an 80 year old; of those I know in my age group, most can chalk up a few, or more, with only one or two that I can think of maddeningly unscathed.

You get used to it. And that surprises me. You get used to diminishment, to a body that is stalled, an impediment. An alter ego is amazed, aghast perhaps – myself in the roaring 40s, when robust health was an assumption, a given, something you barely noticed because it was always there. Acceptance has set in, somehow, has crept up on you, which is just as well, because the alternative – perpetual rage and resentment – would not help matters. "In 70 or 80 years a Man may have a deep Gust of the World. Know what it is, what it can afford, and what 'tis to have been a Man." Reading Sir Thomas Browne, today I'm in touch with a former self, who was discovering Urne-Buriall in Oxfordshire in the 1970s: "The treasures of time lie high, in Urnes, Coynes and Monuments, scarce below the roots of some vegetables." Today, I am warming to Browne's discussion of the long view: "such a compass of years will shew new examples of old things, parallelisms of occurrences through the whole course of Time, and nothing be monstrous unto him, who may in that time understand not only the varieties of Man, but the varieties of himself, and how many Men he hath been in that extent of time." Yes, yes – exactly. And how strange, how exciting, to find an echo of what I have been thinking about myself in that wonderful 17th‑century mind.

And it is of the varieties of myself that I am aware, seeing how today's response to Browne links me to that Oxfordshire self, in mid-life, busy with children, but essentially the same person. The body may decline, may seem a dismal reflection of what went before, but the mind has a healthy continuity, and some kind of inbuilt fidelity to itself, a coherence over time. We learn, and experience; attitudes and opinions may change, but most people, it seems to me, retain an essential persona, a caste of mind, a trademark footprint. A poet's voice will alter and develop, but young Wordsworth, Tennyson, Larkin are not essentially adrift from their later selves. There is this interesting accretion – the varieties of ourselves – and the puzzling thing in old age is to find yourself out there as the culmination of all these, knowing that they are you, but that you are also now this someone else.

Simone de Beauvoir confronted the problem in her extensive study Old Age: "Old age is particularly difficult to assume because we have always regarded it as something alien, a foreign species: 'Can I have become a different being while I still remain myself?'" What is at issue, it seems to me, is a new and disturbing relationship with time. It is as though you advanced along a plank hanging over a canyon: once, there was a long reassuring stretch of plank ahead; now there is plank behind, plenty of it, but only a few plank paces ahead. Once, time was the distance into which you peered – misty, impenetrable, with no discernible landmarks, but reassuringly there. In old age, that dependable distance has been whisked suddenly behind you – and it does seem to have happened suddenly. Not long ago, there was some kind of balance – a fore and aft, as it were. No longer; time has looped back, regressed, it no longer lies ahead, but behind. It has turned into something else, something called memory, and we need it – oh dear me, yes, we need it – but it is dismaying to have lost that sense of expectation, of anticipation. The mind does not always keep up – the subconscious, rather. In dreams, I am not always the self of today; I am often young, or younger, and if my children are present they have often become children again, obligingly – we have all jumped backwards. The mind cannot bear too much reality, it seems; in the same way, my husband Jack is nearly always present in my dreams – it is 12 years since he died, but at night he returns, not always recognisably himself, but a shadowy dream companion figure that I always know to be him.

When De Beauvoir published Old Age in 1970 she was 62, so from my viewpoint she was barely even on the approach road to the status itself. And, indeed, her own life experience is hardly cited at all in her long, densely researched, somewhat impassioned and rather wonderful book, which remains an illuminating investigation of the subject. She embarked on this, she says, because she saw a conspiracy of silence about old age, as though all were in denial, refusing to anticipate their own future, and, in consequence, choosing to ignore the situation of the old. She wanted to explore the way in which old age is not just a biological but a cultural fact. She ploughed through history to see how the old had got on, from classical Greece to the present day, she pursued evidence of attitudes towards old age. She searched out the voices of the old. The book is laced with references from art and literature, from sociology, psychology, philosophy. She died at 78, so she got there herself – in 1986 you would certainly be considered old at 78.

Whether or not she turned the analytic eye on her own old age, she had already had a grim experience of it, managing the care of Jean-Paul Sartre in his wretched decline into infirmity and blindness. A harsh diminishment, for these two intellectual heavyweights, and nicely reflecting all that she had written of the alienation from oneself that is the condition of old age.

I am sharing old age with friends, but not with a partner. In that, my situation is entirely average: three in five women over 75 in this country live alone. The men go first. Jack knew that, and expected it; after his retirement, he spent much time organising our affairs, and would talk routinely of a future that excluded him, to my irritation. I would remonstrate, and he would smile amiably: "Statistics …" Well, he was right – though cheated, statistically, since he died at 69. The world is full of widows – several among my closer friends. We have each known that grim rite of passage, have engaged with grief and loss, and have not exactly emerged but found a way of living after and beyond. It is an entirely changed life, for anyone who has been in a long marriage – 41 years, for me: alone in bed, alone most of the time, without that presence towards which you turned for advice, reassurance, with whom you shared the good news and the bad. Every decision now taken alone; no one to defuse anxieties. And a thoroughly commonplace experience – everywhere, always – so get on with it and don't behave as though you are uniquely afflicted. I didn't tell myself that at the time, and I doubt if it would have helped if I had, but it is what I have come – not so much to feel as to understand.

A common experience – like old age itself, for those fortunate enough (if that is the right word) to get there. Here we are – the eightysomethings – around 1.4 million of us in the UK, most of us with nothing much in common except the accretion of years, a historical context, and a generous range of ailments from which we have probably been allocated two or three. For each of us the experience is different, each of us endures – or challenges – it differently. Both endurance and challenge will of course be more successful from the vantage point of financial security, and if you are not too encumbered on the ailment front.

Old age costs; it costs the nation, it costs those going through it. We contribute nothing, but require maintenance – a winter fuel allowance, free TV licence, bus pass, free prescriptions, all the kindly state indulgences. Those don't add up to luxury, for anyone, any more than the state pension does other than provide basic subsistence. And old age has its needs, its greeds. You may not yearn for a Caribbean cruise – I don't – but certain comforts have become essential, the accustomed perks that make daily existence a bit more than just that. I can't start the day without a bowl of the right kind of muesli topped with some fruit and sheep's milk yoghurt; I can't end it without a glass (or two) of wine. I need the diversions of radio and television. I want flowers in the house and something tempting to eat – these are greeds, I think, rather than needs. And – high priority – there is reading, the daily fix, the time of immersion in whatever is top of my book pile right now. As demands, requirements, all of this is relatively modest. Much of it – the reading, the flowers – goes back to prelapsarian days before old age. The difference, though, is that then there were further needs and greeds, and those seem to have melted away, to have tactfully absented themselves as though to make things a bit easier because they would indeed be an encumbrance now.

Out with acquisition, excitement, and aspiration except in tempered mode. And, on another front, I don't in the least lament certain emotions. I can remember falling in love, being in love; life would have been incomplete without that particular exaltation, but I wouldn't want to go back there. I still love – there is a swath of people whom I love – but I am glad indeed to be done with that consuming, tormenting form of the emotion.

So this is old age. If you are not yet in it, you may be shuddering. If you are, you will perhaps disagree, in which case I can only say: this is how it is for me. And if it sounds – to anyone – a pretty pallid sort of place, I can refute that. It is not. Certain desires and drives have gone. But what remains is response. I am as alive to the world as I have ever been – alive to everything I see and hear and feel. I revel in the spring sunshine, and the cream and purple hellebore in the garden; I listen to a radio discussion about the ethics of selective abortion, and chip in at points; the sound of a beloved voice on the phone brings a surge of pleasure. I think there is a sea-change, in old age – a metamorphosis of the sensibilities. With those old consuming vigours now muted, something else comes into its own – an almost luxurious appreciation of the world that you are still in. Spring was never so vibrant; autumn never so richly gold. People are of abiding interest – observed in the street, overheard on a bus. The small pleasures have bloomed into points of relish in the day – food, opening the newspaper (new minted, just for me), a shower, the comfort of bed. It is almost like some kind of end-game salute to the intensity of childhood experience, when the world was new. It is an old accustomed world now, but invested with fresh significance; I've seen all this before, done all this, but am somehow able to find new and sharpened pleasure.

Those of us not yet in the departure lounge and still able to take a good look at what has made them – us – like this can find some solace in doing so. What has happened is such an eccentric mixture of immediate and long-drawn-out, the arrival of a condition that has been decades in the making but seems to have turned up this morning. The succession of people that we have been – Sir Thomas Browne's "varieties of himself" – are suddenly elided into this (final?) version, disturbingly alien when we catch sight of a mirror, but also evocative of a whole range of known personae. What we have been still lurks – and even more so within. This old age self is just a top dressing, it seems; early selves are still mutinously present, getting a word in now and then. And all this is interesting – hence the solace. I never imagined that old age would be quite like this – possibly because, like most, I never much bothered to imagine it.