Men are more than twice as likely as women to be sexually active in old age but good health is the key for both to feeling naughty, says a study published Wednesday by the British Medical Journal.
Doctors looked over two big probes into the health of the American population.
One survey covered 3,000 people aged 25-74 who filled in questionnaires in the mid-1990s as part of an investigation into midlife.
The other survey, focussing on old age, was carried out a decade later among a similar number of volunteers aged 57-85.
At the age of 55, men have on average almost 15 years of sexually active life ahead of them, and women 10-and-a-half years, the researchers found.
They also discovered a major gap between the genders on sex lives.
"Overall, men were more likely than women to be sexually active, report a good-quality sex life and be interested in sex. These gender differences increased with age," according to the paper online.
The biggest gap was among 75- to 85-year-olds, where 38.9 percent of men said they were sexually active, compared with 16.8 percent of women.
Another 41.2 per cent of the men were interested in sex, compared with 11.4 percent of the women.
Within the "sexually active" group of the 75- to 85-year-olds, 70.8 percent of men rated their sex life as of good quality, compared with 50.9 percent among women.
Why such a difference?
It could be partly explained by opportunity, say the investigators.
Around three-quarters of men across all age groups said they had a partner.
Among women, though, only two-thirds of respondents between 25 and 54 had a partner. For women aged 75 and beyond, fewer than four in 10 had a partner - a figure reflecting women's longer lifespan and the tendency of men to marry younger women.
Good health, too, was vital for sexual wellbeing, said the study.
An individual in sound health is almost twice as likely to be interested in sex and can expect to enjoy around six more years of sexual activity compared to a peer in poor health.
Authors Stacy Tessler Lindau and Natalia Gavrilova, from the University of Chicago hope the findings will help end a taboo.
"Doctors rarely address sexual concerns in older adults, particularly in women," they say.
In a commentary, Patricia Goodson, a professor at Texas A&M University, said the news that US adults can enjoy "many years of sexual activity beyond age 55" was good news.
But there were also intriguing questions, she said.
If the study's measure of "sexually active life expectancy" ia credible, American men generally stop having sex around the age of 70, about eight or nine years before their death, according to demographic life expectancy.
For women, "sexually active life expectancy" would ended around 65, yet their demographic life expectancy was around 82 or 83.
"The measure sheds no light on the intriguing - and still poorly understood - question of why, even though they enjoy fewer years of a sexually active life, many women do not perceive this as a 'problem'," said Goodson.
"Neither does the measure provide details on how women and men manage, attempt to enhance, or deal meaningfully (and uniquely) with their ageing sexuality."
The survey carried out in 1995-6 defined sexual activity as having had sex with at least one partner in previous six months, whereas the survey from 2005-6 defined it as having sex with at least one partner in the previous 12 months.
Respondents who had sex two or three times a month or more were defined in both surveys as having sex regularly. Between 95.0 and 97.8 of respondents described themselves as heterosexual.