It’s almost summer, and the latest competitive body fad to come out of the United States (hot on the heels of M-obama arms) is the awesome middle-aged bikini body. Everywhere you turn, fortysomething celebs are stripping off — for magazine covers, awards ceremonies (the American comedienne Kathy Griffin, 48, recently accepted a gong wearing nothing but a tiny two-piece), or just on the beach in full view of the paparazzi — and challenging us to find a flaw in their perfect 10 physiques.

Nicollette Sheridan, 45, of Desperate Housewives, is one of the most photographed bikini challengers — you can’t keep her off the sand; Demi Moore, 46, does it Baywatch-style, with a surfboard under one arm, and Elle Macpherson, 46, makes it look as easy as hanging out in jeans and a T-shirt. But don’t kid yourself that these women (and scores like them) just happen to be holidaying, hoping for a suntan. This is all about deliberately seeking exposure to remind the public, casting directors or ex-boyfriends that age is merely a number. Besides, bikini excellence is this season’s personal quest. People magazine featured the soap star Valerie Bertinelli on a recent cover, and she has just turned 49. Who can push the bikini cutoff date the furthest is the story of the moment.

Did it start with a naked Jennifer Aniston on the cover of GQ? Maybe, but the bar has drifted higher, and now not only is 40 a ‘So what?’ age to be in great shape, but entry qualification for membership of the Fabulous for Her Age club is tougher. As of now, fortysomethings are required to have a sculpted figure that makes the one they had in their twenties look sort of sloppy and noncommital. Getting away with a bikini is no longer an option — it’s about having a body so toned, so lean, so taut and blemish-free that you could wear it to a party. And the older you are, the more defined the body has to be.

You may think there’s pressure on famous young women to look like swimsuit models — Princess Bea was picked over in the press not long ago — but the pressure on women old enough to be their mothers has the unmistakeable edge of do or die about it. Last year, passing the bikini challenge was desirable; now, it’s more a matter of survival. For obvious reasons, the bikini represents the final frontier of body competition. It’s not only the semi-nudity, it’s the harsh direct sunlight, the absence of figure-enhancing props, like high heels, and the fact that on the beach, a body can be papped from any and every exposing angle. Out on the glaring hot sand, there is nowhere to hide, no Spanx to smooth out those porridgey saddle bags, no black opaques to cover up the knee-surgery scars. That’s why the bikini reveal is a challenge that separates the Got-its from the Not-quites. It’s now a requirement of women who want to stay visible.

The rules are simple: you bare all, you get papped and you score points according to how you do on the day. Beach wraps and sarongs are not officially banned, but in these circumstances, modesty is a poor excuse for covering up. (So, be warned, Nicollette. Keira can wear a kikoi, but if you knot a sarong around your hips, it looks suspiciously like a safety net. And you’re only 45.) Somehow, Kim Cattrall gets away with the long-line cardigan and sandals, but since her style is indistinguishable from Samantha’s, and Samantha couldn’t go swimming without a picture hat and Jimmy Choos, it’s allowed. Cardigans or not, the new beach body needs to have been put through its paces like a marine’s. You must be able to cross your legs in a deckchair and guarantee that those thighs won’t so much as shiver. You need to have a stomach without a hint of crepiness, and a cleavage that won’t crease. The slightest hint of an uneven tummy or bottom, and it’s game over, if not career.

The standard is set by Elle “The Body” Macpherson, who, at 46, has a figure to rival Giselle’s, only more rock solid. The other frontrunner is Cindy Crawford, 43, who says she’s happier with her body now than she was 20 years ago, because she’s worked for it: “When I was 23, other women looked at me and said, ‘She’s never had kids.’ But now I have.” In other words, any 23-year-old can look hot in a bikini, but wind on 20 years and only a crack elite can say the same. And that’s what this is all about: having the determination, the time, the money, and the original God-given assets, to leave the competition in the dirt.

Anyone approaching 40 will already appreciate that what you took for granted requires more effort as the years go by. Cindy stays in shape with the help of a personal trainer, daily workouts and the zone diet, and that’s on top of the model genes and a lifetime’s commitment to looking after her body. It will be the same story with Nancy Shevell, Paul McCartney’s girlfriend, Sharon Stone and Courteney Cox Arquette — discipline, denial, work, work and more work.

It was tempting to think this might catch on in the way other celebrity habits, like Botox, have, but on this occasion we mere mortals are going to have to sit it out and gaze in awe as the titans of celeb world challenge each other to the beach walkoff. And good luck to them.

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