A glimpse of something shocking has long

ceased to be, well, shocking. Now it's age that catches our eye. Kathy Marks on the latest users of underwear as outerwear.

It is an arresting image. A 56-year-old woman in a plunging brassiere gazes down from a billboard, in a pose reminiscent of Eva Herzigova in the infamous Wonderbra advertisements.

This time the message is political, not sexual. The charity Age Concern is making a point about our perceptions of older women as part of its campaign against age discrimination at work. The slogan on the poster reads: "The first thing some people notice is her age."

It is, nonetheless, an unsettling sight. True, this is an exceptionally attractive and well-preserved 56-year-old who could easily shave a decade off her age. But she patently lacks the bloom of youth of the stereotypical lingerie model, and thus subverts the Herzigova ad - which is what Age Concern intended.

There is an intriguing subtext to this poster beyond its call for equality. Its shock value lies in the fact that it features an older model, not that it shows a woman in a revealing brassiere. So ubiquitous are such portraits now that we barely raise an eyebrow. Fashion, advertising and evolving social attitudes have combined to anaesthetise us to images that were once confined to the bedroom.

Style may be ephemeral, but it reflects more profound cultural changes. So when Madonna appeared in an exaggeratedly pointy bra designed by John Paul Gaultier in the early 1990s, it was not just a fashion joke; it caught the imagination. The bra, after all, is a sexy and beautiful garment which clothes the sexiest and most beautiful part of the female body. The Gaultier bra, together with similar confections by Vivienne Westwood, coincided with the relaunch of the Wonderbra, embraced by legions of admirers for its push-up and cleavage-enhancing properties. Soon women were turning up at nightclubs and art exhibitions wearing little more above their waists than a frothy black bra.

The "underwear as outerwear" trend appears to be here to stay.

Dawn French, the generously endowed comedienne, turned up on TFI Friday last week wearing a big white bra over her T-shirt, and invited members of the audience to remove it with one hand.

Marcelle D'Argy Smith, editor of Woman's Journal magazine, says: "Women want to show off their bodies in a way that they never did in the past. It is no longer shocking to see a woman wearing a skimpy bra with a transparent blouse on top; it is not even deemed inappropriate at work. And it's not necessarily a 'come hither' thing; it is very pretty and feminine."

Tongue at least partly in cheek, she adds: "We really do have the best of both worlds now. We can wear what we want, and if a man comes too close, we can throw the sexual harassment rulebook in his face."

It was all very different in 1914, when Mary Phelps Jacobs, an American socialite, decided that she was fed up with her uncomfortable corset and, with the help of her French maid, tied two handkerchiefs together with pink ribbon. Thus was the early brassiere born. But it was not until 1925 that it was designed with individual cups and adjustable straps, and only in 1938 were variations in cup size introduced.

Manufacturers and advertisers realised long ago that the bra is much more than just a functional scrap of lace and cotton. It is, in fact, a garment that is central to women's self-image at every life stage. In adolescence, girls are desperate to get into their first bras in order to prove their feminine credentials. Early boyfriends are irrevocably associated with fumblings to get them undone. The start of a more mature affair necessitates the purchase of something new and erotic. Pregnancy signals the start of a bewildering succession of ever more voluminous versions. Breast-feeding means those clever ones with unhookable cups for easy access.

Dr Martin Skinner, a social psychologist at the University of Warwick, says: "The history of art and fashion reflect the changing ways in which women's breasts have been covered, revealed and accentuated through the ages. By definition, the bra is an icon of femininity. That's why bra- burning by early feminists was such a potent symbol; the bra was seen as an object of restraint and repression. Now I suppose you could say that women have given in to constraint in the name of freedom."

If modern feminists can wear lipstick without being accused of treachery to the cause, as the writer Natasha Walter asserted in a recent book, The New Feminism, then they can, too, wear provocative bras with no qualms. And they do wear them, if the success of lingerie stores such as Agent Provocateur is anything to go by. Even Marks & Spencer, which once had an extremely straitlaced range, has introduced whole racks of skimpy and slinky numbers.

British women, once lambasted for their reluctance to splash out on underwear, particularly compared with their Continental counterparts, are spending twice as much on it now as a decade ago. The bra business alone is worth pounds 500m a year. "More and more women are prepared to spend that bit extra," says Jill Kenton, manager of Rigby and Peller, corsetieres to the Queen, whose made-to-measure bras cost up to pounds 400.

Breast sizes, too, have grown larger over the years, thanks to healthier lifestyles, better nutrition and the contraceptive Pill. In the past 10 years, the average bust has advanced from a 34B to a 36C. Manufacturers have responded with larger-cup ranges. The success of Sophie Dahl, the voluptuously-proportioned supermodel, suggests that fashion may be turning away from women with ironing-board figures.

And towards older women? The other interesting point about the Age Concern poster is that it suggests that the nubile young girls who rule the roost in fashion and advertising may not, after all, have a monopoly on allure, that post-menopausal, wrinkled women are equally legitimate sexual beings.

This was the signal that artist Melanie Manchot sought to send when she plastered huge posters of her 66-year-old mother, dressed in her underwear, on hoardings outside a busy Underground station in London last month. Sexualisation of the older woman may be starting to catch on. The charms of veteran actresses such as Helen Mirren, 51, are regularly lauded. Dazed and Confused, the men's magazine, published close-up shots of women in their sixties, seventies and eighties, clad only in their underwear, in its issue last month.

The increasingly explicit, unashamedly sexy advertisements of recent years have given the bra a great deal of exposure and helped it emerge from beneath layers of outer clothes. In some ways, this is a welcome development. Perhaps a garment that defines women's shape more than any other - and is also a powerful symbol of femininity, evoking both motherhood and sexual attraction - should not be coyly concealed.

And if Pearl Read, the Age Concern model, has managed to push the boundaries that little bit further, she may be just the right woman for the job. Ten years ago she brought a ball in Berkeley Square to a standstill when she suddenly unfastened her halter-neck top and gave her astounded fellow guests an unfettered view of her breasts.