Whatever happened to Tom Cruise? Yes, I know there was somebody impersonating him in London's Leicester Square on Monday night -- a chubby, cheesy, eager-to-please labrador puppy; eternal student type, with a bad haircut.

That's not the real Tom Cruise. The real Tom Cruise is the moody Top Gunner; the sexy, smart Charlie Babbit in Rainman; the scary, stunning killer in Collateral.

Except, of course, that isn't the real Tom Cruise at all. Those are the parts he's played. His acting skills, the script and the stuntman have convinced us all that he's eternally young, vital, desirable, indestructible. And that's the trouble. He believes it and is loath to move on.

Of course, it’s nice that he wanders around for hours at premières, letting everyone take his photo, but isn’t it all rather childish? He seems to be saying: “Look at me. I’ve still got it.”

Well, we are looking at you, Tom, and we don’t think you have got it any more. You’re a 45-year-old married man and father of three. Isn’t it time you acted your age?

Tom’s not the only movie star ageing sadly. Look at Hugh Grant – but not too closely. At 47, he was pictured earlier this month being clambered over by some very pretty girl students. And did we think: “Wow! Lucky them. He’s so attractive?” No.

We thought he looks just like that smutty lecher of an uncle, who always gets drunk at family gatherings.

Both are playing out their mid-life crises in public. If they were accountants, they wouldn’t have this problem.

No one expects accountants to stay eternal babe magnets.

But movie stars such as Cruise have an image to maintain.

They’re engineering photo-ops of public mobbings that make them look desperate rather than desirable. But it won’t be long before someone puts the knife in. The surgeon’s knife.

It wasn’t always that way.

Getting “work done” was never a popular option for male stars in the past. If you’d approached John Wayne, armed with nothing more than a tube of moisturiser, he would have snarled: “Get off your horse!”

Now there is a bizarre belief that you can fool people into believing you’re not getting older by cutting bits out of your face.

Robert Redford was photographed emerging from a restaurant recently, with a face not so much “lived in” as vandalised. Comedienne Joan Rivers claims he’s had “bad” work done – and she should know. But he denies it. “I'm not afraid of ageing,” he has said. “It’s a fact of life, unless you go to some length to arrest it, and that’s not me.”

He has acknowledged that his “pretty boy” days are over.

“Nobody is swooning over someone my age,” he says.

So why do it? What I think happens to these chaps is that they suddenly look in the mirror, at 50-plus, and think: “Crikey! What happened? Make me look young again!”

The result is that sudden and radical “wind tunnel” look – not so much youthful as utterly scary. Take the Douglases père et fils. Have Kirk and Michael really done themselves any favours in their desperate attempts to stay young? Those oddly wide, wrinkle-free eyes, the unnaturally smooth cheeks.

It’s not remotely sexy. In fact, it’s rather pathetic.

Actresses know better. Their careers are largely dependent on their looks and from early on, they take action, be it careful maintenance (facials, treatments, peels etc) or subtle surgical intervention. Meryl Streep (58), Michelle Pfeiffer (49) and Demi Moore (44) have taken care of their faces and now look fantastic.

Some actors are smart and enter their “father of the bride” years with relish. Look at Jack Nicholson. Get up as close as you like. He doesn’t care. He’s recently enjoyed two of the best parts he’s ever had – both written for a pensioner.

He sent himself up delightfully in Something’s Gotta Give, as a man with a young girlfriend who then falls in love with her mother, the fabulous Diane Keaton (who, at 61, plays a maturing sex symbol to perfection). In About Schmidt, he shared a hot tub, in a hilarious scene, with the frighteningly naked Kathy Bates.

It’s because he’s confident in his skin that he’s one of the few ageing stars who we could quite easily believe younger women would find irresistible. Like Sean Connery and Dustin Hoffman, Nicholson didn’t become famous just for his looks.

And we’ve seen female stars who got to the top with a fierce intelligence and talent, rather than just a cute butt – Susan Sarandon, Jodie Foster, Helen Mirren, Catherine Deneuve – ageing much more happily than their “babe” peers.

Certainly, it is hard for Cruise and Grant et al. They live and work in a fantasy world. We believe – and, sometimes, they believe themselves – that they are like the people on the screen: fearless, witty, irresistibly sexy, able to run up mountains, stop a moving train and save the world.

But they’re not. Why can’t they accept it and avoid ageing sadly?

- Jan Etherington