Stella McCartney shows that 40 is the new 30

A 40th birthday party was once a milestone for all the wrong reasons. Officially past it, you could let yourself go and begin the long slide towards decrepitude.

Stella McCartney's 40th demonstrated a very different modern truth. In her sparkling dress, her skin glowing, her hair pulled back into a simple bun, the birthday girl looked not only beautiful but entirely at ease with herself in a way that she hasn't for years.

And she wasn't the only one. Kate Moss, 37, was glamorously ravaged in a sexy black jumpsuit.

There was little jewellery on display and no cleavage. Instead, the female guests flaunted toned, tanned backs and rivers of glossy hair.

Their make-up was barely visible but their smiles were enormous. They looked as though they were genuinely enjoying themselves.

And why not, when all of them, having emerged from the dowdy chrysalis of motherhood, look so much better than they ever have?

Of course, it's no longer a revelation to anyone, apart from Hollywood producers and advertising executives, that middle-aged women are still attractive and stylish. But this glossy posse appears to be improving with age.

Fellow party guest Gwyneth Paltrow, 38, is a case in point. Until now, she's never seemed entirely comfortable in her own skin, working numerous looks over the years with varying degrees of success. We've had the preppy ingénue, the hippy chick, the yummy mummy and that brief post-baby period of inappropriate flirtation with killer heels and tiny skirts.

Now, you can't characterise her style: she simply looks great. Groomed, grown-up and much sexier and more confident than she ever did flashing the Paltrow pins on the red carpet.

At the launch of the Coach flagship store last week, she too wore a simple, backless black jumpsuit, accessorised only with swathes of well-kept golden hair. Beside her, Daisy Lowe, 22, despite all the advantages of youth and beauty, looked a scruffy mess with her fussy maxi dress and tousled mop.

Across town the same night at Vogue's Fashion's Night Out, Sam Cam was flying the flag for the sophisticated sisterhood in a plain green shirt dress that made the starlets, with their barely-there dresses and make-up overload, look risibly try-hard.

Or take Rachel Weisz, 41 (who recently declared that turning 40 was "so not a major milestone").

Her new husband may look like a walking mid-life crisis with his scruffy jeans and his permanent scowl, but she radiates poised elegance, whether she's dressed up in cocktail frocks or down in jeans.

Meanwhile, mother of three Claudia Schiffer, 40, having left the catwalk behind her, has emerged after years of maternity as a fully-fledged knitwear designer, running up costly cashmeres for other well-heeled yummies like herself to wear on the school runway.