I am incandescent with righteous indignation - fired up in a way that puts me in the same boiling blood group as someone who'd stomp off to nail a list to a door or to throw tea into a harbour.

I never expected to find myself feeling this way. It came upon me while I was writing Instructions for Living Someone Else's Life, a novel that I intended to be entirely about the tricky nature-nurture issue. My lead character, aged 25, wakes up one morning to find that he's 43. The exact reasons for this are too complex to go into, but it got me thinking a lot about how this reflects what really happens to men. They're older, but they don't feel it. And the world greets this with three ghastly, deceitful, oppressive and pernicious words: male midlife crisis (MLC).

“Ah... so you're having a midlife crisis, then, Mil.” No, I am not having a midlife crisis. In fact, I maintain that the thing is a largely fictitious entity, like griffins or a decent self-assembly wardrobe. Even more important, if such a thing as an MLC does exist, then it's the biggest misnomer since noming began.

Listen. If I'm in my forties (which I am), and if I dye my hair (which I do), is this symptomatic of a midlife crisis? You wouldn't be alone if you said “Lord, yes”. Quite a few people have winked and whispered, “Hiding the grey, Mil?” to me recently. But I've been dyeing my hair for most of my life. It has been pillar-box red for about seven years and, before that, blue, orange, green, purple, yellow - and everything in between.

The only association between me dyeing my hair and an MLC is that people now make that association. Now, if this were about me and my silly hair alone it would be unimportant. But it's not - it's a near ubiquitous principle. A 45-year-old man buys himself a sports car: MLC. Hold on...he has probably wanted a sports car since he was 7;it has simply taken him 38 years to be able to afford one.

A 42-year-old man starts to take a concerted interest in his appearance: MLC. Hold on...he probably wanted to look acceptable at 25 too, but at 25 you needn't do anything, really (whereas at 42 your skin begins to hint that an over-vigorous wash could easily result in your face falling off into the sink). Almost anything a man does in his forties will be identified and mocked as the sign of an MLC.

OK, it's easy to sneer at the cosmetic because our Zeitgeist has got “looking young” and “being young” fatally entangled and the jowls of a 20-year-old are desired because they subconsciously promise the immortality of a 20-year-old. “I can't be dead - I moisturised.” Yet let's not get confused here. You're not confirming your soulful depth by having sagging stomach fat, rambling eyebrows or Brillo pad tangles of hair erupting from your ears. It's perfectly reasonable for a fortysomething man, alarmed that his body is cartwheeling into chaos, to react in the same way as a twentysomething would: if such a thing ever happened to twentysomethings.

It's prejudice. Against men. Oh yes - let me make that bit unmistakably clear. A 40-year-old woman, seeing that she has let herself slide a little, begins aerobics or yoga and takes up some new, self-asserting activity - salsa dancing, say, or a university course. She is rightly applauded for her spirit and vim. The 40-year-old man in precisely the same position, who suddenly joins a gym and starts playing guitar in his (reformed, now middle-aged) band, is, however, a snigger-whipped target of open mockery.

Take my friend's wife Jane (to protect her identity, I'll call her Cathy). Cathy's husband, Dave (actually Phil), recently started running. She announced this with much eye-rolling to all of us as poor Dave stood in the centre of the room like a criminal being publicly shamed. “Guess what? Dave's started jogging. Midlife crisis time!” Well, let me tell you, Dave used to do cross-country running many years ago. He was a natural runner. But now, mid-forties, he hasn't run for ages and has really piled on the pounds. So he has started running, has he? Thank God. His doctor probably wept with delight.

If midlife crises do exist, and this is an example, then where's the crisis? It's actually a blessing. In this case, it's probably the salvation of Dave's arteries.

But, in almost every case, it's a wholesale boon that should be greeted with a street party and the ringing of church bells. Men often sink imperceptibly into life's sucking mud in their thirties. They can let the descent continue, and most likely accelerate, or they can shake themselves awake and swim upwards.

“But what about affairs, Mil?” I hear you roar. “Suddenly going off with a younger woman - that's your MLC classic, isn't it?” Excellent: I've been waiting for you to fall into that trap. Men are drawn to sleeping with attractive women. Many women find “powerful” men attractive. But men are more likely to be “powerful” - managing director, say - in their forties. The boss wants to sleep with the attractive young secretary now that he's 45, just as he would if he were 17 or 108. If they're both 20, it's just an affair. If he's middle-aged and she's young, it's called an MLC. Like everything else when it comes to MLC, it's a matter of timing and probability. You might as well say that men wearing glasses as they get older is a symptom of MLC; they're trying to look intellectual. Tch.

Bottom line. The average fortysomething who starts regular exercise will probably become fitter than he has been for ten years - quite possibly fitter than he has ever been. Instead of a steady, terminal slide into cardigans and National Trust gift shops, he can burst out into ten or perhaps 15 years of splendid and challenging loveliness with as much vigour and ability as at any point during his entire life. He can use his financial security to explore new hobbies and interests, instead of putting on slippers and waiting for the hearse to arrive. He can even, if he wants to, dye his hair.

You have, middle-aged men of the world, a future. A marvellous future, glittering with possibilities; all you need to do is grasp it through the nettlebed of silly, jealous gibes that society has planted. Do it. And, everyone else, if you know a middle-aged man, then buy him a nasal hair trimmer, and an ab trainer, and cheer him on. Instructions for Living Someone Else's Life (Orion, £10) is available from Times Books First for £9.50, p&p free. Phone 0870 1608080 or visit timesonline.co.uk/booksfirst

Middle-aged men in numbers

50% The proportion of US motorbike fatalities the occur in the over-forties

40 The age at which more men divorce than any other, according to National Statistics

£81 The average annual amount spent on shirts by men aged 30-50

13% The proportion of 40 to 50-year-old men who download music every week

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