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.
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.
Men more sexually active than women in old age
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.
Men think sex lives "good quality"
The biggest gap was among 75- to 85-year-olds, where 38.9% of men said they were sexually active, compared with 16.8% of women.
Another 41.2% of the men were interested in sex, compared with 11.4% of the women.
Within the "sexually active" group of the 75- to 85-year-olds, 70.8% of men rated their sex life as of good quality, compared with 50.9% 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.
