Do you remember The Matrix? It was big a couple of years ago. I think I quoted it in this very space — something about red pills, blue pills, and how far down the rabbit hole you want to go. It was part of the lingo for a while. But the dogs bark and the pop-culture caravan moves on. Anyway, a while back an interview in the Guardian with the film’s composer happened to catch my eye. Usually, when an interview with a guy who’s big on the electronic “dance music” scene catches my eye, my eye promptly glazes over, but not on this occasion. Rob Dougan, an Aussie who lives in a cool loft on the south bank of the Thames in London, had been asked post-Matrix to remix some Sinatra tracks for today’s market — add some hip-hoppish electronica here and there. Unfortunately, he liked the records pretty much as they were. He took a crack at “That’s Life” and was told his remix was not “modern” enough. So it was back to the old drawing board. And then Mr. Dougan observed:

In Sinatra’s time it was really cool to be 50, to be a man. You put on a hat and a suit and you keep on going until you die. Now you get 50-year-old guys in sleeveless T-shirts, going to the gym and desperately trying to fix their hair, and you think: “Whatever happened to real men?”

Well, maybe they had hormone treatments. Victor Davis Hanson recently concluded that “the generic American male accent” was dying out and had been replaced with something affectedly “metrosexual” with “a particular nasal stress, a much higher tone than one heard 40 years ago . . . a precious voice often nearly indistinguishable from the female.” As for the old-school males, wrote Professor Hanson, “I watched the movie Twelve O’Clock High the other day, and Gregory Peck and Dean Jagger sounded liked they were from another planet.” Diana West has written a whole book on this theme: The Death of the Grown-Up. But it rings more plaintive coming from Rob Dougan, a cutting-edge type with his own pop-cult cred on the line. I suspect he may be thinking of Sinatra circa 40 more than 50, but in a way that makes his point: If you look at almost any movie from what we might call the old days, the guys appear older than they are. Almost all the leading men — Humphrey Bogart, Gary Cooper, William Powell — seem designed for eternal middle age: You put on a hat and suit and you keep going until you die.

Now we have youth culture: what’s young, what’s hip, what’s hot, what’s now. The folks who commissioned those Sinatra remixes won’t care if they sound squaresville two years from now, just as all the hep cats ABC demanded as “special guests” for Frank’s 80th-birthday salute in 1995 are now Trivial Pursuit answers. (Salt N Pepa? Hootie and the Blowfish?) Who cares if your hot young act cools off? Something even younger and hotter will be along in a moment.

But what if it’s not? I write a lot about the demographic decline of the Western world, and readers often respond, “So what? Tokyo’s pretty crowded. It’d be kinda nice to have 20 percent fewer people.” Maybe. But the 20 percent who aren’t around won’t be the coots and codgers; the missing folks will be the children who were never born, and the few who were but decided they didn’t want to spend their lives in a joint so tilted toward the geriatric. The eternal adolescence of contemporary pop culture is merely the most obvious example of how society’s self-image is invested in its youth. In star movie roles, everybody’s young. Not necessarily ridiculously young, like Dr. Christmas Jones, the nuclear physicist played by Denise Richards a couple of Bond films back. But young nevertheless. Because young people go to the movies and they don’t want to look at old people.

But in Japan and Europe a generation or two down the road, everyone will be old. Will they still want to look at young people? And, if they do, will they even be able to muster enough young people to star, write, direct, compose the theme music? Or will there no longer be enough youthful energy in society to maintain youth culture’s endless parade of novelties?

Right now, Hollywood movies make more money abroad than they do at home. So, if you wonder about the stuff playing at your multiplex in Des Moines, relax: It’s made for young Asian males, and you’re merely a peripheral market. Perhaps that situation will continue indefinitely, until everyone on screen looks like Dr. Christmas Jones, and everyone in the European and Canadian and Japanese audiences looks like Ethel Barrymore’s grandmother. Two years ago, Alfonso Cuarón made a comically inept film of The Children of Men, P. D. James’s dystopian novel about a world in which people are not merely disinclined to breed (as in latter-day Europe) but literally unable to. The movie looked like a movie — which is to say that everyone in it was young: young heroes leading young gangs pursued by young cops and young soldiers. Thus did Mr. Cuarón miss the point of Lady James’s novel. In the book, youth is in short supply: Paved highways crumble to rutted tracks because the government workers are too middle-aged to maintain the rural districts. Youth is at a premium — as it will be in Japan the day after tomorrow, and Germany the day after that. Yet the boringly conventional casting of the movie unintentionally confirmed the book’s thesis — that a society without the young is so alien to all our assumptions even the adapter couldn’t imagine it.