At age 55, men can expect another 15 years of sexual activity, but women that age should expect less than 11 years, according to a study by University of Chicago researchers published early online March 10 by BMJ. Men in good or excellent health at 55 can add 5 to 7 years to that number. Equally healthy women gain slightly less, 3 to 6 years.

One consolation for women is that many of them seem not to miss it.

Men tend to marry younger women, die sooner and care more about sex, the study confirmed. Although 72 percent of men aged 75 to 85 have partners, fewer than 40 percent of women that age do.

Only half of women 75-85 who remained sexually active rated their sex lives as "good," and only 11 percent of all women that age report regularly thinking about or being interested in sex.

Among those age 57 to 85 not living with a partner, 57 percent of men were interested in sex, compared to only 11 percent of women.

"Interest in sex, participation in sex and even the quality of sexual activity were higher for men than women, and this gender gap widened with age," said lead author Stacy Tessler Lindau, MD, associate professor of obstetrics and gynecology at the University of Chicago.

But the study also "affirms a positive association between later-life health, sexual partnership and sexual activity," she said.

Lindau and co-author Natalia Gavrilova focused on two large surveys, the National Survey of Midlife Development, involving about 3,000 adults aged 25 to 74 and completed in 1996, and the National Social Life Health and Aging Project, involving another 3,000 adults aged 57 to 85, completed in 2006.

Participants provided information about their relationship status and rated the quality of their sex lives and how often they had sex. They also rated the level of their general health as poor, fair, good, very good or excellent.

The results showed that men are more likely to be sexually active, report a good sex life and be interested in sex than women. This difference was most stark among the 75 to 85-year-old group, where almost 40 percent of men, compared to 17 percent of women, were sexually active.

The study also introduced a new health measure, "sexually active life expectancy," or SALE, the average remaining years of sexually active life. For men, SALE was about ten years lower than total life expectance. For women it was 20 years lower.

Men at the age of 30, for example, have a sexually active life expectancy of nearly 35 years, but they can, on average, expect to remain alive for 45 years, including a sexless final decade.

For 30-year-old women, SALE is almost 31 years but total life expectancy is more than 50. So men that age can anticipate remaining sexually active for 78 percent of their remaining lifespan, while women at 30 can expect to remain sexually active for only 61 percent of the remaining years.

The authors conclude that "sexually active life expectancy estimation is a new life expectancy tool than can be used for projecting public health and patient needs in the arena of sexual health," and that "projecting the population patterns of later life sexual activity is useful for anticipating need for public health resources, expertise and medical services."

In an accompanying editorial, Professor Patricia Goodson from Texas University says Lindau and Gavrilova's research is both refreshing and hopeful. She says: "the study bears good news in the form of hope ... the news that adults in the US can enjoy many years of sexual activity beyond age 55 is promising."

Goodson adds that many unanswered questions remain in the field of older people and sexuality, such as problems with measurement and silence regarding the sexual health of ageing homosexual, bisexual or intersexed people. "They stand as dim reminders of the limitations inherent in applying science to the study of complex human realities, and the cultural values shaping the topics we choose to study," she concludes.