Well into Bridget Jones's twenties, the sexual world is her lobster (as Bobby Robson so famously malapropped), but the approach of 30 can seem like the menace of a distant juggernaut coming down the wrong side of a dual carriageway towards her. Yet, for Bridget's older brother Brian, 30 often heralds his period of maximum sexual popularity thanks to money, status and the confidence he once lacked. Just as it was cruel for him to be so powerless in the face of nubility when a teenager - with older, cooler men nicking the girls from his classroom and undergraduate cohort - so now it is cruel for Bridget that her allure is waning and younger women are picking off the few remaining sane, solvent and single men in her social circle.
After 30, a curiously precise inversion has occured in their respective pulling powers. Assuming he is not grossly abnormal, it is now Brian who has by far the greater number of potential sexual partners. Not only will he have good prospects should he pursue women of his own age or older, those in their early twenties or even younger are also a serious possibility if he plays his cards right.
How very different for Bridget. In her late teens and early twenties, nubility meant she was at least in with a chance with pretty much anyone she set her sights on. Increasingly, after the age of 30, with younger men her chances of even one-night stands, let alone the stable relationship she may crave, are terrible - in fact, in a perfect reversal of the statistics, about as poor as Brian's were in his mid-teens of pulling women his own age or older than him.
Worst of all, as her thirties wear on, the number of older men who are available or even quarter-desirable for Bridget shrinks rapidly. Sixty per cent of those who got married stay married, and at best are only good for an affair. Of those who have become divorcees, a gruelly potage of stepchildren, regrets and maritally induced penury do not seem attractive.
But strangest of all for Bridget is something that no one has properly prepared her for. Just as countless impoverished former millionaires and deposed politicians suddenly discover that friends, wives and lovers no longer have time for them, many women are shocked by the extent to which their looks affected others' reactions to them.
A good friend of mine perspicaciously reported the change in her status which accompanied that in her looks: 'Comparing when I was 21 and 41, the amount of attention I attracted when I entered a room was as different as, respectively, a famous actor at the height of their career and an unemployed cleaning lady.
It's a very gradual, very subtle thing, but you slowly realise that a lot of what you thought were authentic responses to "me" as a human being were actually to how I looked.'
Goodness knows how any of this could ever be effectively explained to the young when at school. But it surely should be.
· Next week: male versus female sexual desire in their twenties.