Joy of sex Photograph: Corbis/Corbis
At 48, Philippa is vivacious, attractive and is having the best sex of her life. "It's quite wonderful," she says. "If you'd told me at 28 that this would be happening I'd never have believed it."
But there's a catch. "It's great sex, but it's not with my husband. To be absolutely honest, he's the last man on the planet I'd want to have sex with."
Philippa is by no means the only fortysomething having a great time in the bedroom. In a survey of 2,000 women carried out by Health Plus magazine, 77% said that their sex life was at its best in their 40s; 82% of that age group also said that sex was as important to them as it had ever been. Other surveys echo these conclusions - one carried out in the US, for instance, found that women in their 40s want to have sex more often than younger women. Jane Polden, a psychotherapist who specialises in working with middle-aged women, says it's a story she hears time and again, as does relationship psychologist Susan Quilliam, co-author of The New Joy of Sex. "There's plenty of research that shows sex gets better for women as they get older," says Quilliam. "It's one of the best-kept secrets of women's lives." While our culture constantly associates sexiness with youth, the truth seems to be that it is those of us who are approaching the menopause - our hair greying, skin sagging a little - who are at our sexual peak.
Polden says there is a host of reasons for this. One is simply that, by this age, women tend to have fewer insecurities. "Younger women are much more likely to be obsessed with their appearance, their weight and so on," she says, "and worries about those things sap their self-confidence and get in the way of them enjoying themselves. Older women are more confident of who they are, and it's a deep-seated confidence, which means they're not scared of intimacy, and they're not scared of going all out for what they need to feel satisfied."
The impending menopause is also significant. As it approaches, levels of the so-called "nurturing" hormones - oestrogen and oxytocin - diminish in women, which allows our testosterone to make more of an impact. The theory is that from puberty women are physically wired to be attentive to the needs of others, our bodies priming us to care for children. In our 40s, though, when those nurturing hormones melt away, many women are led to the epiphany that they have been putting their own needs in second place for decades.
"It's an extraordinary moment of realisation for many women," says Polden. "Doris Lessing sums it up very well in her book The Summer Before the Dark. Her fortysomething heroine has an affair, and she says it makes her feel herself for the first time since she was 13. She's felt overwhelmed, controlled almost, by this hormonal surge ... and now it's draining away, and she can work out who she is, and who she wants to be."
For many women, the light that goes on illuminates a faltering marriage. "It was almost inevitable, really, that I'd find Paul boring after 20 years," says Philippa. "But it wasn't just the boredom - he also seemed not to be the right partner for me any more. It struck me that when I was younger I was searching for an alpha male, a provider, but what I want now is a much more sensitive man." From the practical point of view, she says, there is also more space in her life to enjoy herself now that the children are growing up. "It's wonderful to go out for lunch with someone who notices what I'm wearing and flirts with me. It spices everything up, it makes me feel energised and youthful, at precisely the moment I was beginning to feel a bit dull and over the hill." And a lot of women in their 40s seem to share this sense that they suddenly have time to indulge themselves. Having brought up their toddlers, they have more freedom to go out again and relax with their husband, their partner - or indeed someone else entirely.
The data on extramarital affairs supports the notion that, like Philippa, many fortysomething women are having great sex with someone other than their husband. A study last year from the University of New Hampshire found that the most common age for women to have affairs is 45 (for men it's 55) and these US figures dovetail with a UK study last summer that found that women aged 45-54 had the highest rates of STIs for their gender, while men aged 55-60 had the highest rates for theirs. The authors of that last study said that their research suggested that "sexual risk-taking behaviour is not confined to young people".
For those who embark on them, the fortysomething sexual relationship is often a way of reclaiming their youth. "It's been the most delicious, unexpected, delightful pleasure ever," says Nancy, 50, who split up with her long-term partner four years ago and, soon afterwards, met George. "I thought I'd hung up my boots, and to find myself in love again has been amazing. I don't feel 20 again - I can't claim that - but I do feel the world is full of possibilities, just as I did when I was 20." For some it represents a second chance. "I met Stephen at 19 and married him at 22," says Harriet. "He was my only lover. But our marriage went stale, as they do, and one night at a drinks party I met someone new, and we ended up having the most amazing sex upstairs in a bedroom while the party was happening below." Harriet has since found another lover, although she still lives with her husband and their children.
Women's enjoyment of sex in their 40s can also be chalked up at least partly to feminist advances, which have made us much less likely to settle for second best. In the past, many women simply tolerated their husbands' affairs - now they're getting out there themselves. "My marriage wasn't working," says Sian, 49, who has had an on-off affair with an old university friend for the last two years. "And my feminist instinct is that if things aren't right for you as a woman, you change them. Women of my generation know we can change the world - we've done it before." The fact that most women in their 40s are in employment also means that they have many more opportunities to meet men - and to pursue affairs - than their stay-at-home predecessors.
These mid-life affairs don't necessarily spell the end of a marriage. Quilliam believes that a sexual relationship with a life partner, especially one with whom you have children, leaves an attachment even once the sex is over, which can potentially carry a couple through a few crisis years. For some women, though, finding a lover in their 40s is a prequel to leaving their marriage (in seven out of 10 cases, divorce is instigated by women). "Many of those who have affairs at this time in their life are having what we'd call transition relationships," says Quilliam. "Their main purpose is to reaffirm these women sexually and to help them believe that they could have another long-term relationship with someone new - even if it isn't this particular one. So they boost your self-belief, and they give you the confidence to get back on the market sexually."
For Quilliam, another key factor in the 40s sexual peak is that this is the age at which we really start railing against the inevitability of death. "The menopause rehearses our mortality," she says. "We have to face the fact that we're no longer able to have babies, no longer able to pass on life. We're losing our looks: it's a wake-up call to the fact that we won't go on forever, that one day in the no longer impossibly distant future, we're going to die. And sex and death are very closely related. Put crudely, we fuck to prove we're alive."
• Some names have been changed
A new book has claimed that Hollywood biggie Marlon Brando enjoyed two nights of passionate sex with Jacqueline Kennedy.
