Lily Allen pointedly says that her range of generously skirted prom dresses, which went on sale this week in New Look, is aimed at everywoman.

It is to be hoped that no one takes her literally. Dressing the world in a cosy cuddly all-inclusivity is a nice thought, but everywoman dressing is damaging, deluding and getting out of hand. We’ve all been there. That moment on a shopping trip when your accomplice unearths an item that she likes and you really like too. You try it. It fits. It looks good, albeit a very different kind of good from how it looks on your companion. You consider whether it’s an unfair breach of copyright, but decide that a matching pair of waistcoats probably constitutes a genuine bonding moment.

It does not. Even if your shopping mate were a similar age, buying the same outfit is extremely irritating and possibly slightly creepy behaviour. When the shopping accomplice in question is your teenage daughter, it borders on identity theft.

Harsh, especially since sooner or later we all have to cross that rubicon where your body says yes, I look pretty damn fine in this gorgeous, jewelled, sleeveless lookalike Chloé shift from Zara, and damn it, I’ve worked hard to achieve these biceps, and your head points out weakly that looking good is all very well, but whatever happened to looking appropriate?

It’s a dilemma clouded by the fact that many women are hanging on to their looks, muscle-tone and interest in fashion far longer than their mothers did. All power to them. But it means that deciding what looks great and what simply grates becomes an increasingly close call. The Duchess of York wearing something constructed from saloon bar curtains has, arguably, a certain familiar charm. Duchess and daughters in complementary saloon bar curtains carries a whiff of manipulative, parental wrong-headedness that lent the Duchess’s jolly admissions that she and her daughters go on double dates a cringeworthy inevitability.

The Duchess probably means well and is by no means the sole offender.

Years ago I was at a dinner for Gianni Versace where his young niece Allegra, who was about 8, appeared in Mini Me versions of her mother Don-atella’s outfit and hairdo, and was invited to sit with the su-permodels. Again, this was no doubt a well-meant if indulgent gesture by the adults who loved her most. The temptation to strengthen ties with your children by letting them know how much you appreciate their taste (so much so that you want to hijack it) can be overwhelming, especially if you present it to yourself as a form of approval akin to praising them for good Latin test results. But it isn’t the same. “Every generation tries to separate themselves from the previous ones by inventing new slang, new music, a new uniform,” says Marisa Peer, a behaviourial expert and therapist.

“And they get very cross when you borrow their language or clothes. Remember how they all stopped wearing jeans temporarily when Jere-my Clarkson became notorious for his. They don’t want you in their world, wearing their ripped jeans. Learning to separate psychologically from parents is a healthy step in creating their own sense of self and ultimately preparing to leave home.”

Perhaps we instinctively realise that our daughters’ first independent purchases are an early declaration of separation, which is why the urge to match a rapidly blossoming teenage daughter, the compulsion to micro-manage her wardrobe to make her look more sophisticated, more soignée and thereby prompt observers to remark “Surely you are too young to have such a grown-up daughter” and the impulse to encourage her to be more classic/more adventurous/ie, more like you, or the you that you’d like to be, amount to the same thing: a mother who is losing sight of where her identity ends and her daughter’s begins and is fearful of recovering her perspective.

What makes it all the more confusing is the current culture of entitlement. Or, as Nancy Dell’Olio and Carol Vorderman so nearly put it in their fashion TV series that never was, “I’ll Wear What I Bloody Well Like”. Encouraged by programmes such as How to Look Good Naked and Ten Years Younger, and by retailers who have merrily leapt on the lucrative mothers-shopping-with-daughters demographic, there is a growing belief that we can all wear what we like. I was taken aback last week, for instance, by the number of fortysomething women (and articles by fortysomething women) complaining that the Kate Moss range had pieces in it that they couldn’t wear. Ahem, ladies: those micro hot-pants and Glasto dresses were never (despite the PRs’ sales pitch) meant for us.

There are neutral areas in fashion, of course; jeans, T-shirts, straight-legged trousers, ballet pumps, fitted jackets, but as one colleague says: “As a rule of thumb, I never buy the same style of jeans as my daughter. And if there’s a high-street dress we both like that looks good on her, she gets ownership”.

Yet the myth that fashion is one big free-for-all continues to be promulgated. Madonna, a 48-year-old mother-of-three, produces a range of drip-dry wrap dresses and pencil skirts for the teenage haven H&M: Jade Jagger and her teenage daughters are photographed looking like sisters (ones with identical taste in shoes and dresses, moreover); ditto Demi Moore. It often ends in tears. “Fundamentally girls don’t want their mothers to look too glamorous,” says Peer. “They want them to be cosy and baking cakes. My mother was beautiful and I used to pretend my grandmother was my mother.”

Hard though it may be to hand the fashion baton on to your daughter, difficult as it is not to interfere when you think she’s committing a fashion mistake (“never tell her something makes her look fat”, counsels Peer. “Say that the other dress looked amazing on her and only give advice when asked. You must let them wear things you don’t like”), there are rites of passage for them and for us. Besides, there are compensations: older women look wonderful in (and are more likely to be able to afford) well cut, quality designs. And looking on the bright side, if you had only sons, you’d have no bench-mark of what not to wear.

10 commandments for mothers with daughters (and all women over 40)

1. Thou shalt resist Abercrombie & Fitch. It’s soft, it’s comfortable. It’s designed for teenagers.

2. Thou shalt be seen only at the most casual events in hoodies.

3. Thou shalt wear high-tech trainers only in the gym.

4. Thou shalt not show thy political awareness by wearing slogan T-shirts. Thou hast the vote. Use it.

5. Thou shalt wear jeans, but not the identical cut and brands as thy teenage daughter.

6. Thou shalt not wear sparkly body powder even in jest. It settles in the wrinkles.

7. Thou shalt not wear leggings. Period.

8. Thou shalt not suddenly decide to be edgy, although if one has always been an eccentric dresser, carry on as normal.

9. Thou shalt never do mixy-matchy or themed outfits with one’s daughters.

10. Thou shalt treat thyself to expensive classics. And lock them away.

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