We tend to caricature the elderly as either raddled wretches or cuddly Yodas. Illustration by Golden Cosmos Audio: Listen to this story. To hear more feature stories, download the Audm app for your iPhone.

Early in his career, Paul Newman personified a young man in a hurry forced to wait his turn. His go-getter characters infiltrated the old-boy network, wore the gray flannel suit, and toiled away before finally, in midlife, grabbing the brass ring and coasting for home. In “The Young Philadelphians” (1959), for instance, Newman played Tony Lawrence, whose mother, over his cradle, gloats, “Someday, he’ll take the place in this city that belongs to him.” Young Philadelphians, it’s clear, are merely old Philadelphians in the making. While Tony is at Princeton, a silver-haired Philadelphia lawyer so venerable he has a British accent tells him, “I’m confident that in due time you’ll become a partner in Dickinson & Dawes.” As Tony shinnies up the greasy pole at an even more eminent firm, he grumbles when old man Clayton has him work on Christmas and grouses that big clients are “reserved for the seniors” who wear homburgs and smoke pipes. Eventually, though, he makes partner and smokes a pipe of his own. Yay.

Times have changed. In “Disrupted: My Misadventure in the Startup Bubble” (Hachette), Dan Lyons, a fifty-one-year-old Newsweek reporter, gets his first shock when he’s laid off. “They can take your salary and hire five kids right out of college,” he’s told. His second shock occurs when he takes a lower-paying job at a startup called HubSpot, where his boss is a twentysomething named Zack who’s been there a month. Lyons arrives for work in the traditional uniform of a midlife achiever—“gray hair, unstylishly cut; horn-rimmed glasses, button down shirt”—to find himself surrounded by brogrammers in flip-flops who nickname him Grandpa Buzz. His third shock is the realization that the tech sector usually tosses people aside at fifty. A few chapters later, he advances the expiration date to forty. A few chapters after that, he’s gone.

This sharp shift in the age of authority derives from increasingly rapid technological change. In the nineteen-twenties, an engineer’s “half life of knowledge”—the time it took for half of his expertise to become obsolete—was thirty-five years. In the nineteen-sixties, it was a decade. Now it’s five years at most, and, for a software engineer, less than three. Traditionally, you needed decades in coding or engineering to launch a successful startup: William Shockley was forty-five when he established Fairchild Semiconductor, in 1955. But change begets faster change: Larry Page and Sergey Brin were twenty-five when they started Google, in 1998; Mark Zuckerberg was nineteen when he created Facebook, in 2004.

With the advent of the cloud and off-the-shelf A.P.I.s—the building blocks of sites and apps—all you really need to launch a startup is a bold idea. Silicon Valley believes that bold ideas are the province of the young. Zuckerberg once observed, “Young people are just smarter,” and the venture capitalist Vinod Khosla has said that “people over forty-five basically die in terms of new ideas.” Paul Graham, the co-founder of the Valley’s leading startup accelerator, Y Combinator, declared that the sweet spot is your mid-twenties: “The guys with kids and mortgages are at a real disadvantage.” The median age at tech titans such as Facebook and Google is under thirty; the standard job requirements in the Valley—which discourage a “stale degree” and demand a “digital native” who’s a “culture fit”—sift for youth.

That culture is becoming the culture. At Goldman Sachs—a century-and-a-half-old investment bank that is swiftly turning into a tech company—partners are encouraged to move on after five years or so, or risk being “de-partnered.” As one senior banker says, “There’s always somebody on your six”—military terminology for the guy right behind you. A recent A.A.R.P. study revealed that sixty-four per cent of Americans between forty-five and sixty had seen or experienced age discrimination at work. Accrued eminence still matters at law firms and universities (though tenured positions have fallen fifty per cent in the past forty years), but the rest of the culture has gone topsy-turvy. Even as Lycra and yoga make fifty the new thirty, tech is making thirty the new fifty. Middle age, formerly the highest-status phase of life around the world, has become a precarious crossing. The relatively new tech sector is generating enormous amounts of a very old product: ageism.

“Ageism” was coined in 1969, two years after the Federal Discrimination in Employment Act set forty as the lower bound at which workers could complain of it. The upper bound continues to rise: the average life span grew more in the twentieth century than in all previous millennia. By 2020, for the first time, there will be more people on Earth over the age of sixty-five than under the age of five.

Like the racist and the sexist, the ageist rejects an Other based on a perceived difference. But ageism is singular, because it’s directed at a group that at one point wasn’t the Other—and at a group that the ageist will one day, if all goes well, join. The ageist thus insults his own future self. Karma’s a bitch: the Baltimore Longitudinal Study of Aging reports, “Those holding more negative age stereotypes earlier in life had significantly steeper hippocampal volume loss and significantly greater accumulation of neurofibrillary tangles and amyloid plaques.” Ageists become the senescent figures they once abhorred.

The baldest forms of ageism include addressing older people in “elderspeak”—high, loud tones and a simplified vocabulary—and tarring them with nouns like “coot” and “geezer” or adjectives like “decrepit.” The young can’t grasp that most older people don’t feel so different from their youthful selves. When Florida Scott-Maxwell was living in a nursing home, in 1968, she wrote in her journal (later published as “The Measure of Our Days”), “Another secret we carry is that though drab outside—wreckage to the eye, mirrors a mortification—inside we flame with a wild life that is almost incommunicable.” She felt like the person she’d always been. Last year, Americans spent sixteen billion dollars on plastic surgery, most of it on fountain-of-youth treatments for wrinkles, trying to close the gap between interior vitality and exterior decay.

Eye tucks get an eye roll in two books that view the problem not as the elderly but as a culture that has forgotten how to value them. Ashton Applewhite’s “This Chair Rocks: A Manifesto Against Ageism” (Networked Books) and Margaret Morganroth Gullette’s “Ending Ageism, or How Not to Shoot Old People” (Rutgers) both grapple thoughtfully with how we got here. Yet each writer tends to see ageism lurking everywhere. Gullette, a resident scholar at the Brandeis Women’s Studies Research Center, is given to such pronouncements as “Typically, anonymous old people portrayed in art exhibits, websites, and journalism convey decline ideology.” That’s a lot of terrain to cover with a “typically.” Applewhite, an activist whose blog, “Yo, Is This Ageist?,” fields inquiries on the topic (the usual answer is yes), is the more grounded guide. She begins by suggesting that we call the elderly “olders.” Ordinarily, this sort of cream concealer—“aging” replaced by “saging” or “eldering”; Walmart greeters hailed for their “encore career”—deepens the frown lines it’s meant to erase. But Applewhite’s point is that older people may not be qualitatively different from “youngers.” She notes that only ten per cent of Americans who are at least eighty-five live in nursing homes, and that half of those in that cohort don’t have caregivers; for the most part, she maintains, they are cognitively robust, sexually active, and “enjoy better mental health than the young or middle-aged.” Her conclusion: “Clearly, hitting ninety was going to be different—and way better—than the inexorable slide toward depression, diapers, and puffy white shoes I’d once envisioned.”