Rowan Pelling: When did teenagers start dressing like this?

'Style' used to be the great retreat of middle-aged women. But what if your friends' daughters do it better than you?

I recently bought myself a pair of black, knee-length breeches from Oasis's vintage-look range that clung like a second skin. The fabric and cut were classy, but what really persuaded me was that, two years shy of 40, I could squeeze into such a scrap of cloth in the first place. Even so, as I preened in front of the changing-room mirror, a snide little voice inside my head whispered, "Just because Madonna can, doesn't mean you should." "But I must and will have them," I hissed back at the little voice. "They'll look bitching with my gold stack-heeled disco sandals and sequinned cape." "Yeah, for all those occasions for when you and Beyoncé go clubbing," sneered the voice. "Am I bovvered?" I replied, and went to seek more constructive advice.

A couple of listless schoolgirls trying on camisoles said, "Yeah, they're OK," while the twentysomething shop assistant said, "Like, cropped trousers aren't really my thing, but those are cool." "But am I too old for them?" I wailed. The three young women looked faintly embarrassed, as though I had blundered into a socially unacceptable topic such as openly enquiring about somebody's colostomy bag. "I mean, I dunno," said the assistant. "It kinda depends how you wanna look." I suddenly realised that something halfway between Christina Aguilera and Barbara Cartland was not a helpful answer. If I didn't know how to dress my age, these youngsters were hardly likely to show the wisdom of Solomon on the subject. I mean, when did you last see someone of my generation looking or acting like a grown-up? Certainly not Kate Moss or Sadie Frost, who were both photographed on their respective hols this week dressed like teenagers and dating boys barely out of short trousers.

This week I had a brief scout round the January sales in Cambridge. All around me teenagers fought shoulder-to-shoulder with yummy mummies for the same slash-priced garments. It's become clear in recent years that the shops that now do best on the high street either aim squarely at the perennially girlish mum (Jigsaw and Monsoon), or are those where 18- to 48-year-olds shop side by side, casting sly glances at one another to reassure themselves they're on to something hot. Young women know the loaded, professional females are eagle-eyed at spotting classy designer rip-offs and older women know the girls will soon winkle out the hottest street trends. Topshop leads the pack by several furlongs, but Zara, Oasis, Hennes and Gap aren't far behind.

With the post-baby-boomer generation's pathological reluctance to surrender to advancing years, long-accepted style dictates have been abandoned - but nobody's got a clue what the new rules are. Fashion writers continue to dictate that skinny jeans and short skirts should never be sported by anyone over the age of 25, but all this really means is that the writer is smugly 22 or that her own thighs are too chunky to suit such garments. The fact such advice is utter bollocks can easily be demonstrated by a) Kate Moss b) an army of 35-plus women in Gap skinnies and riding boots c) a quick flick through Heat. Who looks better in hot-pants: Alison Goldfrapp or Charlotte Church? I rest my case.

Style, as opposed to merely being fashionable, is the mature female's prerogative - or used to be, in any case. The bad news is that the kids are now scrapping for our turf. Once upon a time, women of 40 yearned to look like teenagers; now teenagers strive to emulate 40-year-old Sarah Jessica Parker (pictured). Girls in their teens and early twenties are dressing with a sophistication and ambition my generation entirely lacked. Just as I finally felt I was getting my look together, it's being worn far better by all my friends' daughters. Take my beloved, beaded antique-look Topshop cape (a snip at £30). I wore it to a neighbour's house for drinks and her 23-year-old daughter, Sophie, opened the door looking ravishing in the same garment teamed with a Helen Storey little black dress. "My cousin and her friend have got it too," she said helpfully.

If further proof were needed that the old sartorial order had collapsed, just take a look at Newsnight. I switched on the box on Thursday to see what the programme's political editor, Martha Kearney, had to say about Charles Kennedy's booze problem and found myself calling to my husband, "Blow me, she's had a makeover and that's an Issa dress!" I also suspected a chemical peel and some fillers, but I leave other lovers of politics to draw their own conclusions. I patiently explained to the husband that clingy Issa frocks are usually sported by the likes of Keira Knightley and concluded that it's only a matter of time before Kirsty Wark goes on air in Roland Mouret's "Galaxy" frock and Jeremy Paxman's getting nasty in a hoodie.

Dressing to suit your inner child is a liberating trend and thus highly contagious. I mean, Martha Kearney looked like a milkmaid of 20. All I ask is that when we mere mortals try to follow in the celebs' footsteps that we get a little sound advice en route. High-street chains should recruit sensible but fashionable oldies who will fearlessly tell us when we look like Scarlet Johansson from the back and Jessica Tandy from the front. The kind of Kabbalah-approved gurus who would find a way of saying to Madonna, "Quite frankly, Madge, the leotard and fishnets are de trop." Who would tell Nicky Haslam that he looks like Miss Havisham in Robbie William's clothing and should get his sorry stubble and sagging butt down to Gieves and Hawkes pronto. Someone who can tell me if the real problem is that I'm just a little too young for those Oasis breeches, but that in 10 years' time they'll look fabulous.