"Robin Hood," one of the biggest movies in America right now, is all about how Robin got his start. He begins as an archer, and by the finish, he has just begun his life in the Sherwood Forest. He has assembled his men, but he has not yet stolen anything from the rich or given a scrap to the poor. Everything is ahead of him. And he's played by Russell Crowe, who is 46 years old.

Now go back to 1976, to "Robin and Marian." Robin Hood returns from the wars, his glory days behind him. He's beginning to feel his advancing years, and Maid Marian lives in a convent. Everything, or just about everything, is behind him. And he was played by Sean Connery, who was 45 years old.

We're seeing this more and more in movies, not actors playing younger than they are, but rather actors playing their age - middle age - as a time for beginnings. Look at the "Sex and the City" women, who are in their 40s and 50s playing women in their 40s and 50s, yet their whole atmosphere is young, and their whole story is one of constant renewal. There's no sense of settling down or turning from the world.

Jennifer Aniston is in her 40s and doesn't pretend otherwise, and yet she still appears in comedies of courtship that, a generation ago, would have been the province of women 10 years younger. And Matt Damon, who turns 40 this year, still has the aura of a young man going out for his first job interview.

Part of this is simply perception. Baby Boomers and Generation Xers are getting older, and so we look at people in their 40s as young. And because there are a lot of us, we get to set the cultural agenda. It seems unfair that people who, in their youth, made their middle-aged parents feel like Methuselah (and made everyone over 30 feel over the hill) should never get our comeuppance. But there's no denying the strength of numbers.

Yet if it were merely a matter of perception, wouldn't the stars of yesteryear also look young to us when we look back on their films? That would stand to reason, but we know this isn't the case. Look at Clark Gable in "Command Decision" (1948). At 47, he looked like an old man. Look at Ava Gardner in her early 40s in "55 Days at Peking" (1963) or "The Night of the Iguana" (1964). The beauty lingers, but really only as a shadow. The glow is gone, and she could be 10 years older.

To be blunt, Gable and Gardner looked as if they'd already spent decades smoking and drinking, and their lives were beginning to show up on their faces. The same could be said for Lana Turner in "Peyton Place" (1957). She was still attractive, but while still relatively young (36), she had already made the turn into middle age.

Not just perception

So this phenomenon goes beyond perception. Today's stars have a younger aura because they tend to take better care of themselves. Many are still smoking, but they have less of a predilection for pickling themselves in alcohol. They're also working out. Crowe, who has struggled with his weight over the years, got into excellent shape for "Robin Hood." A generation or two ago, no one, except for the occasional cult-of-the-body star, ever thought about lifting weights.

There's yet a third reason screen actors are maintaining an aura of youth into middle age: attitude. There's a refusal to get old - to some degree it's a refusal to become mature - that's just part of our culture. Katharine Hepburn wasn't an alcoholic, but at 44 she was practically an old lady in "The African Queen" (1951). Meanwhile, look at Tom Cruise as a senator in "Lions for Lambs" (2007). The first time you see him, you think, wait, he's too young to be a senator. But Cruise was 45 then, the same age as John F. Kennedy during the Cuban missile crisis.

'Peter Pan' nightmare

Actually, there's something to be said for the very adult, been-to-hell-and-back quality that JFK exudes in "Crisis," Robert Drew's behind-the-scenes documentary of the Kennedy administration. You don't see that quality nowadays, not on screen or in our public life, which brings us to the downside of middle age today: Adulthood just isn't what it used to be. In fact, when I see movies like "Sherlock Holmes" or "The Losers," I wonder if we're not lost in some "Peter Pan" nightmare, in which adult characters can behave like children, and yet no one seems to notice.

Let's be fair to the past. Ava Gardner may have been practically an old woman at 41. But in "The Killers," at 23, she was more of an adult than most of our current actresses will ever be. Gable may have been an old fat guy at 47, but at 31, in "Red Dust" (1932), he was a man. Not a young man. A man. He was a year younger than Ashton Kutcher is today.

Kirsten Dunst is 28 - the same age as Greta Garbo in "Queen Christina" (1933) - and yet she's still an ingenue. At 32, Hilary Swank tried to act the femme fatale in "The Black Dahlia" (2006) but seemed like a girl playing dress up. Meanwhile, Jane Greer - perhaps the sexiest, slinkiest and scariest film noir heroine of them all - was only 22 when she filmed "Out of the Past" (1947). And Jean Harlow was only 26 when she died. She was a woman from her first appearance onscreen.

Perhaps it takes a Depression or a World War II to put miles on people's spirits and make them seem older. By comparison, later Boomers and Generation Xers have lived their lives in unchallenging times. I'm not complaining - that's a good thing - who needs calamity? Who needs to feel or act old a minute before it's necessary?

Yet I wonder: Maybe we're seeing in our buoyant, middle-aged stars a representation of our own consciousness - the unclouded consciousness of a people who have evaded life's deepest and most meaningful lessons. That would even be worse than aging, to go through life and miss the point.

E-mail Mick LaSalle at mlasalle@sfchronicle.com.

This article appeared on page E - 1 of the San Francisco Chronicle