Interviews with more than 7,000 over-50s found we tend to be more sexually satisfied over the age of 80 than when in our 50s, 60s or 70s.
Couples become closer, with stronger emotional bonds and growing sexual compatibility.
Although people tend to have sex less often as they get older, that sex is likely to get better as couples progress through retirement and into old age, the researchers found.
And they discovered that women, in particular, find it easier to become aroused in their 80s than in their 60s or 70s.
The scientists, from the University of Manchester and Manchester Metropolitan University, suspect this is partly because relationships strengthen with age, meaning even in old age couples can get to know each other better.
But it might also be because retired couples do not have the distractions and stresses of middle age, when they might also have children still at home and elderly parents of their own to look after.
The researchers, whose report is called ‘How Long Will I Love You?’ also found people were happier if they had a more active love life, right into their 90s.
Those who kissed, cuddled or touched intimately at least once a day had a higher ‘life satisfaction’ score than those who only did so two or three times a month, the academics found.
And as they aged they tended to report being emotionally closer, be more likely to share their partner’s sexual likes – and yet at the same time feel less obliged to have sex.
Baroness Sally Greengross, chief executive of the International Longevity Centre, said: ‘We know that many men and women remain sexually active throughout their lives, and that intimate relationships in later life can continue to have a positive impact on overall health and wellbeing.
"Unfortunately, in 2017 there is still a need to dispel myths around relationships in later life."
Research author Dr David Lee, of the University of Manchester, said: "We know that positive sexuality and intimacy throughout the life course is linked to higher levels of happiness and wellbeing – irrespective of age."
He stressed that all couples are different and some people’s sex lives may dwindle as they get older, particularly if they have health problems. But he said on the whole sexual happiness increases. "Part of this may be because the longevity of a relationship may be playing a role – they have known each other a long time and be more likely to know what works for them," he said.
Having fewer work and family responsibilities may also play a part for some couples. Dr Lee said: "Some people said their sexual lives were curtailed in their 50s because they still had children at home.
"Sometimes quality of life improves as you get older."
But he added: "But a lot of this is going to be offset by poor health."
Dr Lee said the findings contradict society’s belief that the elderly do not have sex lives, an assumption that means they do not receive good sexual health care. "Older people have a right to good sexual health care and should be able to easily access joined up services to help them meet that goal," he said.