What does it feel like to be old? Not middle-aged, or late-middle-aged, but one of the members of the fastest-growing demographic: the “oldest old,” those aged eighty-five and above? This has been the question animating me for a couple of years, as I’ve tried to write a novel from the perspective of a man in his late eighties. The aging population is on our collective minds; a statistic that intrigued me is that the average life expectancy in the U.K.—and, by extension, most of the rich West—is increasing by more than five hours a day, every day. I’m in my mid-thirties, but felt confident that I could imagine my way into old age. How hard could it be, really?

Somewhere along the way, though, things went wrong. My protagonist became Generic Old Man: crabby, computer illiterate, grieving for his dementia-addled wife. Not satisfied to leave him to his misery, I forced on him a new love interest, Eccentric Old Woman: radical, full of energy, a fan of wearing magenta turbans and handing out safe-sex pamphlets outside retirement homes.

In other words, I modelled my characters on the two dominant cultural constructions of old age: the doddering, depressed pensioner and the ageless-in-spirit, quirky oddball. After reading the first draft, an editor I respect said to me, “But what else are they, other than old?” I was mortified, and began to ask myself some soul-searching questions that I should have answered long before I’d written the opening word.

The first was: Why did I so blithely assume that I had the right to imagine my way into old age—and that I could do it well—when I would approach with extreme caution the task of imagining my way into the interior world of a character of a different gender, race, or class? Had I assumed that anybody elderly who might happen to read the book would simply be grateful that someone much younger was interested in his or her experience, and forgive my stereotyping?

The conundrum of who has the authority to write about old age is that, unlike the subjective experience of most imagined Others, seniority is something that many of us will eventually experience for ourselves. By contrast, I can imagine what it might be like to be a man, for example, but won’t ever know for sure. As the literary scholar Sarah Falcus writes, building on the work of Sally Chivers, “We may all have a more mobile relationship to age than to other perspectives or subject positions … because we are all aging at any one moment.” Yet just because I may, one day, know if I got it right—perhaps, to my surprise, I will find the world of my own old age populated entirely by grumpy old men and old women who are either lost to dementia or sprightly and renegade—doesn’t mean that I should be cavalier about how I imagine my elderly characters now. Of course, like any fictional representation, old age can be done well or badly regardless of one’s own positioning as an author, but there’s less chance of being called out on hackneyed depictions of old age, in part because those in the know—the over-eighty-five-year-olds themselves—haven’t historically had any cultural power.

Stereotypes of old age, whether positive or negative, do real harm in the real world, argues Lynne Segal, the author of “Out of Time: The Pleasures and the Perils of Ageing” (2013). She says that the biggest problem for many older people is “ageism, rather than the process of aging itself.” There is no possibility of diversified, personal approaches to aging if we are all reductively “aged by culture,” to use the age critic Margaret Morganroth Gullette’s iconic phrase, from her 2004 book, “Aged by Culture.” Gullette highlights the limitations of having only two socially accepted narratives of aging: stories of progress or stories of decline. Neither does justice to the “radical ambiguities” of old age, Segal says. We’re forced either to lament or to celebrate old age, rather than simply “affirm it as a significant part of life.”

Old age is perplexing to imagine in part because the definition of it is notoriously unstable. As people age, they tend to move the goalposts that mark out major life stages: a 2009 survey of American attitudes toward old age found that young adults (those between eighteen and twenty-nine) said that old age begins at sixty; middle-aged respondents said seventy; and those above the age of sixty-five put the threshold at seventy-four. We tend to feel younger as we get older: almost half the respondents aged fifty or more reported feeling at least ten years younger than their actual age, while a third of respondents aged sixty-five or more said that they felt up to nineteen years younger.

The researchers also found “a sizable gap between the expectations that young and middle-aged adults have about old age and the actual experiences reported by older Americans themselves.” Young and middle-aged adults anticipate the “negative benchmarks” associated with aging (such as memory loss, illness, or an end to sexual activity) at much higher levels than the old report experiencing them. However, the elderly also report experiencing fewer of the benefits that younger adults expect old age to bring (such as more time for travel, hobbies, or volunteer work).

These perceptual gaps between generations are large and persistent. Simone de Beauvoir, in her exhaustive study “The Coming of Age” (published in 1970, when she was sixty-two), wrote, “Old age is particularly difficult to assume because we have always regarded it as something alien, a foreign species.” The anthropologist Barbara Myerhoff, who made the documentary film “In Her Time,” about a community of elderly Californians, when she was in her forties, believed that “we are dehumanized and impoverished without our old people, for only by contact with them can we come to know ourselves.”

Even more confusingly, we don’t experience old age identically. As Germaine Greer puts it, “Nobody ages like anybody else.” The poet Fleur Adcock, who is eighty-one, says “this great range of abilities and states of health confuses the young: they can’t figure us out.” We age as individuals and as members of particular social contexts, yet the shared experience of old age continues to be overstated. The eighty-two-year-old British novelist Penelope Lively writes that her demographic has “nothing much in common except the accretion of years, a historical context, and a generous range of ailments.” At the same time, though, she warns that aging is such a “commonplace experience” that nobody should “behave as though … uniquely afflicted.”

The actress Juliet Stevenson, who is in her late fifties, recently commented that “as you go through life it gets more and more interesting and complicated, but the parts offered get more and more simple, and less complicated.” The same could be said for the dearth of good roles for old characters in literature. Lively believes that “old age is forever stereotyped … from the smiling old dear to the grumbling curmudgeon.” In fiction, she says, the stereotypes “are rife—indeed fiction is perhaps responsible for the standard perception of the old, with just a few writers able to raise the game.”

I started to realize that, in creating my spunky elderly female character, I had romanticized the version of old age that tells a story of progress, indulging a fantasy of who I might be when I’m old. When writing her, I had been thinking of Jenny Joseph’s “Warning,” regularly voted the U.K.’s favorite postwar poem, in which the young speaker imagines with longing the freedoms of rebellious old age and the prospect of making up for the “sobriety of youth.” I’m hardly a renegade now, however, so why did I harbor the illusion that as I get older I will somehow throw off the shackles of propriety? Most of what has been written in the sociological literature about life in our seventies, eighties, and nineties suggests that who we are when we are old remains pretty close to who we were when we were young. There is comfort in the idea of some consistency of self across the decades. While sometimes distressing, the denialism of old age—think of the sixty-three-year-old Freud’s horror at realizing that the elderly gentleman he’s glimpsed on the train is in fact his own reflection, or the scientist Lewis Wolpert’s lament, “How can a seventeen-year-old like me suddenly be eighty-one?”—is also proof of our ability to remain on intimate terms with younger versions of ourself. “Live in the layers, / not on the litter,” as the Stanley Kunitz poem goes, and he knew what he was talking about: he became Poet Laureate of the United States at the age of ninety-five.