Image copyright Thinkstock Image caption Coupe or toupee? Just some of the 'solutions' to mid-life crises

Instead of fretting about hair loss and diminished virility, should mid-life be re-evaluated as a time to try new experiences and re-invent oneself? Stephen Smith, 55, sets out on a mission to answer these eternal questions.

On his 80th birthday, the French statesman Clemenceau was taking the air on the Champs-Elysees with a friend when a beautiful young woman came towards them. As she passed by, Clemenceau turned to his companion and sighed, "Oh, to be 70 again!"

There are only two subjects, according to the film director Peter Greenaway: sex and death. Put them together and you have the mid-life crisis. On the one hand, the waning of one's charms, vigour and fertility; on the other liver-spotted paw, the incessant susurrus of sand through Father Time's hourglass.

Of course, we prefer to avert our thoughts from such elemental things, and tell ourselves that the mid-life crisis (MLC, for short) is about concerns like status and goals. I have a hunch that these are peripheral.

The Oxford English Dictionary defines the MLC as a "loss of self-confidence and feeling of anxiety or disappointment that can occur in early middle age". But when is that, exactly?

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Stephen Smith, above left, with septuagenarian socialite Nicky Haslam. Smith presents In Defence of the Mid-Life Crisis, on BBC Radio 4 at 10:30 BST, Saturday 20 August. Catch up on BBC iPlayer Radio

Most authorities suggest it begins in the mid- to late-40s, though it's perhaps a state of mind more than a black letter day on an actuarial table.

It struck former rugby league player Danny Lockwood when his hair began to desert him.

Image copyright Danny Lockwood and PhotoLens & DA Calverley Image caption Strands of time: Danny Lockwood in his mulleted heyday and, right, today

"I'd had a mullet in the 80s and this came as a bit of a shock," says Lockwood, now in his 50s. He booked himself into a trichology clinic and emerged, he says, looking like Davy Crockett.

"I got through a meeting and thought 'This ain't right'. So I went to the barber's and asked for a number one - had it all shaved off again. The wife never even saw it."

Anyone who watches sitcom or romcoms will know men are typically the butt of MLC storylines. But the writer and journalist Miranda Sawyer recounts a by-no-means rare example of the distaff MLC.

"The strongest feeling I've had was that I've done it all wrong… I've woken up in this life and it's not my life," says Sawyer, recounting the moment her mid-life crisis truly descended.

She had woken up one day, in her 40s, and felt her life "should have been something different".

Sawyer has documented her experiences in a new book, Out of Time, and is candid enough to admit that mid-life sex can be a ticklish issue.

"As you get older, there's a lot invested in your relationship with your partner and to ask them for more sex, different sex, less sex, better sex, becomes really, really hard."

Image copyright Getty Images Image caption Sawyer felt her life 'should have been different'

Nor is the phenomenon confined to the straight community.

Broadcaster Simon Fanshawe, 59, detects what he calls the gay mid-life crisis when he spots a man d'un age certain in unduly tight shorts.

The MLC for gay men, he says, is inextricably bound up with coming out.

Whether they like it or not, formerly settled married men who come out in their middle years reset their personal chronometers, says Fanshawe.

"One day they're fine and with the wife and children… six months later they've come out and suddenly there's this tattooed leather queen coming down the road. Whatever age you actually come out, in your head you're 16. It's a kind of year-zero of being gay."

You might be interested to know what a world-class philosopher has to say about the MLC, but Alain de Botton (46) wasn't available, so we had to settle for the late Arthur Schopenhauer instead.

Image copyright Getty Images Image caption Lester Burnham, played by Kevin Spacey, in American Beauty, undergoes a traumatic mid-life crisis

The views of this 19th Century German seer were outlined to me by Kieran Setiya, a philosophy professor who is writing a guide to midlife.

"Schopenhauer's basic argument is that the problem with getting what you want is that your pursuit is over and then you have nothing to do," says Setiya. "He thought we were doomed to swing endlessly between the boredom of having no desires, and the agony of having unsatisfied desires."

Happily Schopenhauer managed to get out of the bed the right side one morning and acknowledged that although all desire was ultimately pointless he thought the pursuit of atelic activities, like going for a walk, seeing friends for a coffee, was less likely to lead to depression and futility

I tried to have the best of both worlds, by inviting BBC director general Tony Hall to savour a cup of coffee with me while I leveraged an inflation-busting salary review, but I soon discovered Schopenhauer's iron laws weren't to be trifled with.

Image copyright Getty Images Image caption Pining for his youth, or, failing that, his 70s - former French PM Georges Clemenceau

Someone who gets that very well is former businessman turned stand-up comic Dave Streeter. He had it all, to coin a phrase: family, business, nice house. But then the business went, and pretty soon, so did everything else. Streeter adapted the presentation skills he had learned at work into stage patter.

"My wife got the house, the car, the kids. I got the guilt and a four-man tent," he says. "The tragedy is, I don't know four men that like camping."

Like all the MLC veterans I met, Streeter looks back on it as a valuable if painful stocktaking.

He introduced me to his new passion: vibing, a kind of disco on two wheels. Vibers work up a sweat on static bikes to the sound of club favourites. With the tang of scorched lycra in my nostrils, I reflected that vibing could be seen as a metaphor for the MLC: pedalling furiously, but getting nowhere. But as I essayed a few sclerotic revolutions myself, I understood that, on the contrary, it could be the perfect tonic.

What do Schopenhauer's insights boil down to, after all, but the tried and tested message of the needlework sampler: it's not about the destination, it's the journey.

All the data suggests that we're living longer and beginning to adapt accordingly. What used to be pensionable age is now considered late middle-life; if you're not there yet, by the time you are, it will probably have been recalibrated again, to the bloom of youth - think of all that time you'll have to work on your Pokemon Go handicap.

So to anyone inclined to take a dim view of Clemenceau and the flicker of desire awakened in the octogenarian, all I can say is that he was clearly ahead of his time: 80 is the new 70.

In Defence of the Midlife Crisis, presented by Stephen Smith, is on Radio 4, Saturday August 20 at 1030 BST