Then, in a woman's thirties, she turns effortlessly into the classic dresser: elegant, confident, she wisely invests in a few beautiful pieces each season to slot cleverly into her capsule wardrobe. She never appears twice in the same outfit but her response to compliments is always, "Oh, this old thing? Got it in Paris years ago."
Sadly, for most of us, the second sartorial age remains - like a size 10 body - a nirvana that we pray for but never achieve. Confident only that we are too old and flabby for Lurex boob tubes, we move into the parallel universe of fashion confusion, an era of mistakes, the inability to accept the fact that you are now a size 16, of wearing ugly and unflattering fancy dress suits by day and looking like mutton dressed as lamb at night. Think Sarah Ferguson rather than Princess Diana.
My own wardrobe is an archetype of fashion confusion. Unable to accept my age gracefully, I refuse to throw out a much loved but deeply Eighties pair of DM shoes. I have numerous pairs of trousers that I am too fat to wear but retain in case I start swimming three times a week. Meanwhile, my most recent disaster was an ankle-length red silk dress. It is beautiful, but I already own an ankle-length red silk dress that I have worn once. But, hey, in the false economy of the truly confused it was in the sale.
Luckily, help is on hand from the professionals: Harrods' fashion buyer, Geraldine James; Dickins & Jones' personal shopping manager, Carolyn Robertson; image consultants at Wardrobe and House of Colour; and the divinely classical Anna Harvey, deputy editor of Vogue. Anna - on the day we spoke sporting navy knitted dress by Jean Muir - is so clever and confident that she has been wearing her Jaeger cashmere trenchcoat for 20 years.
The simple, terrible truth is that classic clothes cost more than Lurex boob tubes. "You have to invest in a good fabric that's not going to go shiny," recommends Katrina Oakden, client liaison at the Wardrobe style consultancy. A "good" jacket (good is a very classic word), perhaps a wool and cashmere blend, will cost between pounds 300 and pounds 500; pants (classic dressers never say trousers) will be pounds 100 to pounds 150. "Finish, linings and buttons are all important," adds Anna Harvey, "for which I don't mind paying extra. And I always scrunch an item in my hand for a few minutes when no one is looking to see if it creases. A less than perfect fabric will look hideous."
And before any lost souls claim they can't afford to spend pounds 500 on a jacket, heed the words of Lynn Elvy, director of House of Colour. "Running around the sales getting so-called bargains that you only wear twice is not saving money. The cost of clothes is not what you pay, but the price divided by the number of times you wear them. A good suit should last four or five years, a jacket even longer." So stick that in red silk dress number two.
Lime green was all very well for yesterday but a classic capsule wardrobe must last. "I would stick to black, navy, chocolate, camel and charcoal grey," advises Carolyn Robertson, "then add the seasonal difference with accessories." "Your basics have got to be blue, grey and black," confirms Anna Harvey, "though red can be good, too. I have a scarlet Chanel jacket that is six years old and still gets commented on." So do not, under any circumstances, go and buy a fake dalmation fur coat.
"Don't choose extreme shapes - huge shoulders or epaulettes, for example - that date quickly," says Katrina Oakden. "Same with patterns - a fine check or tweed will last." "And nothing too full or too wide," adds Anna Harvey. According to Carolyn Robertson, a basic jacket that works with both skirt and pants (see, it's easy when you know how), should be single breasted with fairly narrow lapels, not too fitted, cover the hips and stop at the middle thigh. Pants should have narrow legs. Classic designers she recommends include Philippe Adec, Michael Kors, Trussardi, A-line by Anne Klein and Emmanuel by Emmanuel Ungaro.
"You have to learn to understand your body architecture," says Lynn Elvy. "Most women stand in front of the mirror and look at the bits they don't like. So a woman who feels she has big hips will wear a big jumper that shows the world she's putting on weight. In fact, she should recognise her assets rather than cover her debits. If you've got curves, wear them. If you've got angles, wear them." "Get into a size 14," adds Geraldine James, tersely.
Classic dressing doesn't begin and end with a great pant, unfortunately. "Make sure you take your clothes to the cleaner," cautions Geraldine James. "Have a good haircut. Clean your shoes. Look after your nails." Remember, you are no longer a teenager on whom scuffed DMs look charmingly political.
So simple when you know how. And remember, even the professionals aren't perfect. "Last summer," confesses Anna Harvey, "I bought a lavender stretch tube dress from Whistles. Disaster. It's still in the bag."