Life’s defining moments do not always announce themselves with the fanfare of celebration (big birthdays, weddings) or trauma (puberty, divorce). Sometimes they’re like stealth bombers; they come out of nowhere and blow up things soundlessly. Two years ago I experienced just such a moment in the middle of the night. I woke up wanting to go to the bathroom and swung out of bed to stand up. I took a single step in the right direction, then fell to the floor like a plank.

There was the blunt thud of skull hitting wood and the slap of impact that split open the skin on my brow bone. My husband leapt out of bed to put the light on, alert as if the crash had been an intruder. By this time I’d managed to sit up. Blood was dripping from my eye on to my hand and I could feel the throb of nascent swellings at my ankle, hip and shoulder. I remember thinking: “This is the kind of game-changing fall that happens to old people” – bone-breaking, concussion-inducing – not to women in their late forties. I swallowed a couple of painkillers, cleaned myself up and went back to sleep. The following day my eye-socket was a reddish-purple golf ball, lids glued into a slit, and my whole body ached.

In itself, the fall was banal. A clear-cut case of somnambulism; my mind had been awake enough to formulate a conscious intention but the neural pathways of my motor system still slumbered. That day and the next I stayed home, unwilling to suffer people staring. I nursed some angry bruises; but thereafter I got on with things as if nothing had happened.

Looking back, I recognise that the fall registered much deeper. It was sloppy and uncontrolled, as if I were a marionette and someone else, someone malicious, the string-puller. I was a mere player, a pawn, a flimsy vessel bobbing on choppy seas. Worse, like some bizarre prefiguring of my future life, my fall seemed to contain within it every other fall I would henceforth suffer.

From that instant on, I’ve never regained an absolute trust that my body will automatically fall into line with my will: from now on it will falter and fail. I can no longer depend on it to function properly. This, it seems to me, is solid indication that my youth has ended and middle age begun.

These days, when we are persistently told that age is all in the mind, that 40 is the new 30, and 50 the new 40; when entire wings of the cosmetics and medical industries are dedicated to rolling back the effects of passing time; when women are giving birth to first children in their late forties and fifties; when we are all, men and women alike, living healthily for longer, working later and shunning the putting out to pasture we once happily greeted as “retirement”; why, when such things are the new norms, would anyone elect themselves to membership of that most undesirable of clubs, the middle-aged? Shouldn’t I just dismiss my fall as an accident? I still run five kilometres three or four times a week. I work and I parent. I switch and click between being a wife, daughter, mother and friend. I am nowhere near the end of my productive life, as a writer or anything else. And yet I know as surely as day is not night that one season of my life has ended and another begun.

You might ask how I know, indeed, how anyone knows when they’ve arrived at middle age. I’ll admit that it remains fuzzy as to whether middle age qualifies as a biologically distinct phase of life (one that comes with its own neurological and biochemical map) or is just a label we give to a period of mental adjustment that helps us accommodate vague feelings of loss. Then again, perhaps it is merely a socio-cultural construction, no more trustworthy than any marketing category: a shorthand way of dividing people up by their attitudes and lifestyle choices?

When the term “middle age” came into general use in the late 19th century, it was principally in a socio-economic setting. Empire and industrialisation had expanded and enriched the middle classes, and women who had finished raising children could enjoy another decade or two of vigour and relevance. Middle age was actually admired: these women were mature, worldly creatures who had, as the modern saying goes, “freedom to” as well as “freedom from”. The negative tarnish came with the mass production of the 1920s and the theories of scientific management that underpinned it, sharpening our association of youth with productivity and middle age with decreasing efficiency.

You could argue that middle age is thoroughly overdetermined, as Simone de Beau­voir seemed to suggest in The Coming of Age. Writing in 1970, when she was 62, de Beauvoir pushed back against a quiescent society that expected people to grow more “serene” as they grew older. With measured eloquence, she wheeled in whole bodies of literature and philosophy to swat down this idea of resigned acceptance. Instead, she argued, we should accommodate old age through a process of continual, consciously engaged modification. “Life is an unstable system in which balance is continually lost and continually recovered,” she wrote.

This chimes with my sense that we shift this way and that – sometimes literally, as with my fall – before correcting for overzealousness or caution. Though de Beauvoir was writing more about old age than middle age, her labelling of bodily decline, economic redundancy and social marginalisation as important parameters in defining how we age fits with the idea that entering middle age is a kind of subjective reckoning. I’m picturing a Venn diagram that captures the intersection of de Beauvoir’s three factors: middle age is that shady area where the circles overlap. It’s a dappled spot, where the light is fading and the chill of winter starts to set in. The specific age at which we enter this penumbra is different for each of us, but the common quality is a profound sense of alteration and a dawning understanding, dim at first, that there is no point of re-entry to the bright terrain of youth.

In the past year, Penelope Lively, Julia Twigg, Lynne Segal, Anne Karpf, Angela Neustatter and a clutch of their American peers all published books on ageing, attempting to pick up where de Beauvoir left off. These women are a generation older than I am. They’ve been through the wars – menopause, middle age – and emerged unscathed. Now they claim to be wiser, happier, bolder, calmer, more flexible, open and, in some cases, more in touch with youth than before. They offer a relentless good cheer, as if it were permissible to write about late life only by becoming your own superheroine. And they appear to have signed up, one and all, to the delusional idea that you are only as old as you feel.

And it really is delusional. My own mother, youthful in mind as she ever was, would guffaw if, in the face of her ongoing problems with mobility, memory loss and regular if episodic bereavement, I attempted to console her by announcing that 80 was the new 70. In most countries, average life expectancy continues to hover around the late-seventies mark (not far off the Psalmist’s three score and ten) but in developing countries it is much lower. At 82, my mother acknowledges that she’s into borrowed time, like those statistical outliers who live beyond 90 and 100 and skew popular perceptions of this ultimate numbers game. Yes, medicine has increased life expectancy – but not as much, or as broadly, as one might think.

As I gear up to turn 50 this summer what has lodged in my mind is this: that it is a mathematical near-certainty that, with my next birthday, I will have passed the halfway mark. That from now on growing older will be less about marking the age I’ve arrived at than about counting down what is left. At 50 I will quite literally be over the hill; ahead of me, the incline runs downwards. And it doesn’t end well.

Last autumn, some 18 months after my fall, I had a hysterectomy. To be precise, it was a sub-total hysterectomy with bilateral salpingo-oophrectomy. All that’s left is my cervix and I kept that for sentimental reasons. It took weeks to recover from the surgery, during which time I experienced a full-bodied plunge into instant menopause. Joints were popping and bones aching. It was impossible to sleep. Every hour and a half, like clockwork, I’d wake up drenched in sweat, throw off my covers and run to the bathroom window to salute the moon – at least that’s how I now think of my stripped-down attempts at rapid cooling.

A kindly friend put a book called The Wisdom of Menopause into my hands, and I gratefully scurried away to prise out its time-trawled pearls. Sadly, this bestselling book by Christiane Northrup, MD turned out to be an embittered tirade against marriage and family – as if our ties were good only for holding us back, rather than up – and, in the worst tradition of US self-help literature, it was lecturing and strident. More edifying was Jane Shilling’s melancholy and poetic memoir of midlife The Stranger in the Mirror, a book honest enough to acknowledge the effrontery of ageing.

On top of affront, of course, there is grief, bewilderment, alienation, frustration – everything you might associate with being forced to cross a border into a foreign land only to be informed that you can never go back; that your passport has been torn up and your old home ransacked. As a new arrival in this strange nation, I wish to parse the experience before its lessons evaporate or transform. To that end, I have pressed into service oestrogen, my new drug of choice.

Oestrogen is the soft end of age-reversing remedies. It is marketed as “natural” – even though much of it is engineered in the laboratory using horse hormone primers. More slyly, it is billed as a “replacement” therapy not a “supplement”. It replenishes depleted stores, topping up your parched system with nothing more than you already had. Like a debt repaid; you’re entitled to it.

And yet oestrogen’s effects are little short of miraculous. It strengthens nails and bones, boosts energy, lifts libido, makes your skin glow and your hair shine. After taking it for a month, I felt as though I’d been holidaying in Thailand. After two, as if I’d just passed my MOT. And I’ve been evangelising about oestrogen ever since, shamelessly pushing it on friends overcome by fatigue, hot flashes, mood swings and insomnia – friends who, like me, are aghast that instead of gently drifting into midlife, midlife has rudely flung itself at them, exploding like a bag of flour.

Using oestrogen is, however, hallucino­genic. Like taking morphine during labour, it insinuates a languorous pause into an otherwise relentless process. Oestrogen heightens my sense of being at a threshold that demands I make conscious decisions about how to tackle ageing. It is in my power now (for as long as I take the stuff) to call the shots on how rapidly I’m willing to let go of my youth. But what exactly should I do? And where should I draw the line? Choice, however illusory, has entered the equation – and with choice comes temptation.

I can see, for example, how easy it might be to do just a little something. A tiny nip and tuck here, a harmless injection there; a barely noticeable lift, suction or augmentation. These reveries of self-improvement taunt me periodically, though they are quickly checked whenever I come across monstrous images of, say, Madonna, her face distorted by prosthetics or fillers, and the fine line between surgery and butchery is brought home with the thumping finality of a cleaver hitting the block.

Although I can see through such determined resistance to ageing into the inner weakness it betrays, I don’t believe for a minute that the smugness that comes with self-denial is any better – all those go-grey campaigners getting off on feeling superior to women who faff around with hair dye. It’s such a Pyrrhic victory. Unless they ditch their granny-like pieties for the unruly witchiness championed by the likes of Germaine Greer, then I feel they’ve nothing to teach me.

Besides, when I journey down that path of imaginative projection, promising myself I will stop hurling spokes into the spinning dials of my body clock, I find that I’m still far from happy about ageing. I feel unprepared for it. Caught on the hop. Exposed. Most of the time I pretend it isn’t happening, only to be pulled up short by that terrible sense of dissonance occasioned: a) by a chance encounter with a mirror, and b) by friends I haven’t seen in a while, when the unchanging, inner me (source of identity, stability, comfort) is forced to confront a visible exterior that’s been subjected to a Dorian Gray-style makeover.

“You look exactly the same,” a friend I’d not seen in a decade told me recently. “Only fuller.” What stung most was that he did look exactly the same. He didn’t even have the graying temples that supposedly confer “dignity” on middle-aged men. Of course men, accustomed in their prime to greater social and economic power than women, often fall very hard in midlife, not least because there are fewer routes to self-reinvention open to them as they age than reveal themselves to women by way of grandmothering, voluntary work, or the Women’s Institute and its modern analogues of baking, knitting, music or gossip circles. (My mother has developed a whole new eightysomething network through playing bridge, which is a 90 per cent female pursuit, as far as I can tell.)

Lonesome or not, men still manage to remain visible as they age, while women are quietly removed from view, especially in high-visibility professions such as the stage and media. Last year the actress Kristin Scott Thomas was widely reported complaining that in midlife she is no longer seen. “Somehow, you just vanish,” she said. You talk and people affect not to hear you. Or they bump into you in the street. Her disclosure struck terror into the heart of every middle-aged woman I know: if someone as blindingly gorgeous and talented as Scott Thomas could disappear, what hope was there for the rest of us?

The serious point about being invisible is the poverty of viable alternatives. You might think, on the plus side, that if you are beneath regard there is no pressure to conform, or even behave. You can thumb your nose at convention and no one will chide you for it. Like the mischievous old woman in Jenny Joseph’s poem, who promises to rattle her stick along the railings and blow her pension on brandy and fancy gloves, you can make up in midlife for the sobriety of your youth.

But although this – what to call it . . . freedom by omission? – holds out the promise of gay abandon, I’m not convinced that the solution to the painfulness of moving forward is a simple flip into reverse gear. Jenny Joseph’s idealisations of a second childhood (who else but children can be so irresponsible?) are ultimately infantilising. Yet the Loose Women nudge-and-wink alternative of turbocharging sexuality on the other side of fertility feels too much like parody.

The trouble with such attempts to reset the clock is that they play directly into societal pressures that keep women perpetually on the back foot. In our post-industrial society, which demands that we keep redundancy at bay by working ever longer hours for a greater number of years, it becomes imperative to prove that you’re still in the game. That you can keep up with younger colleagues, work nights and weekends. That you can innovate and adapt – else those new brooms will sweep you aside faster than you can say Rip Van Winkle.

I’m not sure how, in this brave new world, where economic efficiency is the true driver behind age-appropriate expectations of how to behave, middle-aged women are supposed to find their way. But I do know that falling is out of the question.

Marina Benjamin is the author of “Rocket Dreams” and “Last Days in Babylon” and is a senior editor on Aeon Magazine. She tweets as @marinab52