Women of un age certain are supposed to rejoice that M&S considers Marie Helvin still billboard-worthy at 56 or that Helena Christensen, 40, has popped up as a pervy pirate in basque and thigh-boots for Agent Provocateur or because Inès de la Fressange returned to the Paris catwalk at 51 without a single person fainting or throwing up.

And fair play to these ladies. Just don't call it progress. It is simply that now all the Grazia gals are bankrupted by their handbag habit, retailers are targeting sensible old trouts with a bit stashed by. If they flash us a face from roughly our own cohort perhaps we'll reward their brave enlightenment by cleaving to their brand.

Yet the only thing that messes with the average female mind more than a bodacious young model is an old one. No sane woman over 35 believes she can look like a dewy teenage fawn. (And having observed lines of them waiting to be coldly appraised at magazine castings - unsmiling, limp, in need of a hug - who would ever want to?) But those doughty veterans - Jerry Hall, those resurgent supers Cindy and Christy - are just stick figures to beat us with. Ageing is not inevitable, their images declare, it is up to you.

While there is much anxiety about how teenagers are rendered anorexic by size-zero fashion plates, older women are reckoned too sensible to be sucked in. Except that according to the Mental Health Foundation a quarter of British women aged 45 to 54 suffer from depression, up by a fifth in ten years. That same age range has seen a rise in alcohol-related deaths and has become the principal female suicide flashpoint. Now I'm a hyper-sceptic of all gloomy stats, but separate studies in Australia and America have uncovered similarly deepening pools of midlife female misery.

Little wonder, perhaps, in this most cruel and judgmental of decades. Once a woman of 63 embraced elasticated waistbands: now she is supposed to despair because she can't rock a red bikini like Helen Mirren.

Once there was little you could do to hold age at bay: now the possibilities are infinite - so what's stopping you then, slattern? The current ad campaigns offer a tantalising deal to fiftysomethings: you can remain visible, as your mother never was, admired and lusted after, you don't even need to lie about your age, but only if, by means of self-punishment and perpetual vigilance, you never grow old.

At a Bafta dinner a few years back, I watched a posse of pretty starlets pretend to chat while slyly looking over shoulders of strappy gowns to gauge their effect on the room. Actresses, I thought, are a perfect paradigm of female insecurity. When we're seeking role models, Hollywood should be the last place we look.

Instead, let us grant women an age armistice, when they cease to be judged on appearance but for who they are. How rarely they get the chance, how magnificent when they do. Since Hillary Clinton, 60, walked into a standing ovation at the State Department she has emitted the rejuvenated vitality of a trillion HRT patches. That freeze-dried campaign smile has thawed into a luminous, I'm-here-at-last beam, undimmed even by listening to David Miliband.

Cleaning up America's international reputation is the political equivalent of asbestos removal: delicate, fraught and highly toxic. No better candidate than a sturdy, dauntless matron like Mrs C. Similarly Iceland brought in Johanna Sigurdardottir as PM, aged 66, to restabilise a country ravaged by reckless men, and Yahoo! took on Carol Bartz, 60, as CEO to heft it out of Google's mighty shadow.

And what a chunk of life these women have behind them: the quirky experience, both professonal and personal, that no journeyman male corporate toiler could emulate. Sigurdardottir: air stewardess, mother, MP, open lesbian, party leader. Bartz: motherless child, homecoming queen, maths geek, cocktail waitress, working mom, cancer survivor, internet chief. All three appointments were a victory not just over a dark cross-cultural distaste for post-menopausal womanhood, but against the dreary forces of same old, same old.

At a conference on the wearying topic of working motherhood, my colleague Camilla Cavendish remarked that the problem wasn't downshifting a career - taking a year off or going flexitime - but upshifting when kids are older or gone. Later a businesswoman in her early fifties told me she'd just joined the approved list of potential Tory candidates: “Getting on the list was easy,” she said. “But I haven't seen anyone selected older than 39.” And a late-forties woman executive on another newspaper said fearfully: “There are almost no women older than me - they seem to disappear.”

Just when a woman comes blinking out of her milky years, ready for action, no longer needed at home, society scans her CV, sees the breeding breaks, the birth date and lobs it binwards.

The other day I said to a close friend: “I don't think I'm as nice as I used to be.” She laughed: “Do you know, I'm not either.” What we meant was that a desire to please prettily, to take the crap and hold our tongues at folly (especially male folly) are over. But why? Maybe it was a decade of child-wrangling bossiness or perhaps you are only born with a set allocation of patience and ours is all used up.

But this week I read a report that proclaimed women have “the best sex of their lives” in their forties. At first I snorted at yet another weary attempt to sex up older women, as if - like Madonna proffering her crotch to the cameras - knowing we are still foxy is the only way to prop up our self-worth. Yet the report suggested that why older women get happier in the sack could be chemical: as our production of caring-sharing oestrogen diminishes and we start pumping out stroppy testosterone, we suddenly start to put our own needs - sexual and otherwise - first. So it seems this being less nice is caused by growing a teeny bit more like a man.

Female empathy, life skills and worldly wisdom combined with male drive: hire that person now! Or at least if the Government is serious about women's mental health, it could launch a national volunteer scheme in which untapped female energy can be channelled. Better than the present proposal to hire yet more state shrinks. Women need to look outwards to keep their sanity. Not always inwards or in the mirror.

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