What a milestone the world passes this Saturday. It's one small birthday for man; one shuddering reality-check for mankind. It's the moment when anyone old enough to remember long-haired youths, or even to have been one, will cry: “Larks-a-whoopsy! Where did the years go?”

Yes, Sir Michael Philip Jagger turns 65. The very notion of Mick, the stroppy, cocky, randy epitome of rebellious Sixties youth, as a senior citizen is hard to believe. But isn't that because he and his irrepressible cohorts are refusing to “grow old gracefully”? Boringly would be a more honest word.

Sixtysomethings always had more leisure time than the rest of us. Now, it seems, quite a few of them have more sex, drugs, rock'n'roll and money as well. After seven kids from four women, Mick is happily ensconced with a lady 24 years his junior. That's no age-gap at all, of course, by the prodigious waitress-pulling standards of his old mucker, Ronnie Wood. But it's not bad for a bloke who has burnt as many candles at both ends as Mick has. And the Stones keep rolling. Pimply pop critics may deride a band that's not so much hip as hip-replacement. But it's still the most durable money-machine in rock. Mick himself is said to be worth only about £215 million, which seems low to me. Yet that in itself suggests a life crammed with deliciously expensive pleasures (and some very costly separations).

But it's not my intent to laud a single man, but an entire generation - of which Mick is but one figurehead. Like them or loathe them (and as someone who grew up in the gloomy Seventies rather than the groovy Sixties, I've always veered towards the latter), the generation born in the 1940s really does seem a breed apart - God's chosen demographic, you might say (if you were a religious sociologist).

Healthier and wealthier than any previous generation, they grew up in an era seeded with hope, revolution and verdant creativity. How much they were favoured by the tide of history and how much they created their own good luck by their feisty resolve to live their lives as they wished will always be debated. If their ridiculed parents had not spent the best years of their own youth halting Nazism, for instance, the rebellious students of 1968 wouldn't have got very far with their sit-ins, let alone their summers of free love. I don't think the mantra “turn on, tune in, drop out” would have cut much ice with the Gestapo.

But whether by happy circumstance or their own determination, Sixties youth was able to tear up the rulebook - in the workplace, bedroom, campus and family life. Not always for the better, we may now think, as we try to cope with the human flotsam washed up by the shipwreck of millions of marriages and the flood of cheap drugs. (There is usually a huge element of hypocrisy involved in being “liberated” about anything, of course, since it nearly always condemns someone else to more physical, mental or spiritual slavery.) But there can be no disputing the impact that this want-it-all generation made on society.

And as Mick so flamboyantly shows, they ain't finished tearing up the rulebook yet. Especially the pages telling you how to behave when you're 45, 55 or 65. I daresay that they will make 85 “the new 35” when the time comes, too. But although the rock aristocracy are the most dedicated disciples of the “18 'til I die” credo, they aren't the only ones intent on packing as much into the seventh decade as they did into their second. A retired teacher recently advertised in The New York Review of Books. “Before I turn 67,” she wrote, “I would like to have lots of sex with a man I like. If you want to talk first, Trollope works for me.” She got hundreds of offers. Clearly there are a lot of Barchester Chronicles fans in Manhattan.

But has that sun-kissed generation redefined lifestyle conventions for all of us, or just for itself? That's a complex question, because for all the huffing and puffing about the Sixties' social revolutions, most of us live just as our parents did: married with kids, mortgaged to the hilt, lashed to the treadmill.

I think the baby-boomers may have been a historical blip: beneficiaries of a temporary generational fragmentation of culture. When most people died before 50, as they did in every era before the 20th century, there was little generational divide. The time-gap between youth, middle and old age was too small. And the world wars of the early 20th century perpetuated that one-generational culture by cutting short millions of young lives. It was only when life expectancy shot up after 1945 that the generations developed their own distinct cultures. (The word “teenager” didn't even exist until the 1940s.) The baby-boomers were the ones to benefit. At each stage of their lives, they could carve their own path.

But since then? I think we are seeing a merging of generational attitudes again. Rock fans are now as likely to be 70 as 17. There are as many fiftysomethings as students networking on the web. Pensioners wear jeans. Twenty-somethings vote Tory. Society is back where it was in the 1930s: a mishmash, but one in which attitudes are defined far more by education, wealth and background than generation.

In their carefree way, Sir Mick and Co have contributed to that U-turn, too. They remind us that membership of the human race doesn't carry with it an obligation to stop enjoying life after you have children, or when you turn 50, 70 or 90. I have no idea what Jumpin' Jack Flash will do to celebrate his first day as a fully-fledged OAP. (Well, OK, I have some idea.) But I bet it won't include leafing through a Saga holiday brochure, or claiming his complimentary freedom-pass trip round England by bus, or joining his local bowls club. And I hope he has two or three decades of majestically satanic misdeeds left in him yet. Because if he can behave like that, so can the rest of us.

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