The short life cycle of the female of the species might be summarised as follows: smokin' hot, strips for glossy mag, dates younger man, covers up cellulite, knees give away true age, dumped for younger model. Even the choicest specimen of sidus umbraculi argentei – the greater-crested movie star – cannot prevent her decline, although there's room for debate about whether she's reached the final stage. Now it's the turn of Demi Moore, who's approaching the dangerous age of 50.

In recent days, speculation about the state of the star's marriage to the 33-year-old actor Ashton Kutcher has reached fever pitch – I suppose it does at least offer a momentary reprieve from "Is muscly Madonna spending too long in the gym?". The 15-year age gap between Moore and Kutcher is, in celeb-watching terms, even more of a delicious disaster-in-waiting than was Madonna's now defunct marriage to Guy Ritchie. Moore looks in pretty good shape to me, and I'm not sure why she's cruelly mocked for following the prescriptions – teeth whitening, manicures, pelvic floor exercises, sun block – that readers of women's magazines are urged to adopt.

Why would anyone go to such lengths to stay young – "viable and fresh, like a lovely frosted cupcake in the bakery of life", in the breathless language of the Daily Mail – if she hadn't been fed a relentless diet of stories about the horrors of getting old? I loathe cupcakes. They leave a nasty taste in the mouth, as does the idea of comparing an adult woman to an infantile, sugary confection.

What's particularly unpleasant about the constant carping about Moore is that she works in an industry where taking care of her appearance is part of her job. (I don't know about her ex-husband Bruce Willis, but I bet lots of male actors quietly use moisturiser.) Even worse is the gleeful biological determinism unleashed by reports of Kutcher's partying in Las Vegas on the weekend of the couple's sixth wedding anniversary, which was supposedly followed by a "torrid sexual liaison" with a 23-year-old.

The age difference between Kutcher and Moore is as nothing compared with the 37-year gap between Rupert Murdoch and Wendi Deng, but the rules are tougher for women. The Mail has waited quite long enough for Moore's marriage to fall apart: "Some will say 'cougar' Demi was asking for trouble in romancing a much younger toyboy who is likely to forsake his partner, sooner or – as seems to be the case here – later, for a younger woman in her physical peak."

You don't often see disgust towards older women expressed so frankly. Yet if Moore's marriage is in trouble, the fact that she and Kutcher are celebrities probably has more to do with it than age; Hollywood isn't exactly famous for enduring relationships. The subliminal message that even a woman as beautiful as Moore can't hold on to a younger man is a way of making other women anxious, even though most of us know couples where the age difference doesn't matter. Scotland's first minister, Alex Salmond, has been married for three decades to a woman who's 17 years older than him, and I haven't read a single article describing Moira Salmond as a cougar.

Wrinkly or predatory? Is that really the choice for older women? I don't think so. And I doubt if comparisons with the animal kingdom tell us anything about the female of our species.

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