Experts said that the new survey provided the first clear and complete picture of sexuality in later life and that it should give older adults a sense of where they stand compared with their peers. Researchers have done previous surveys of sexual activity among older people, but those studies were of patients or other groups who were not nationally representative.

“There’s a large perception out there that sex somehow does not occur in the later years, and this study demonstrates authoritatively that for many people sexual activity does not diminish much at all,” said Dr. Robert N. Butler, president of the International Longevity Center in New York and co-author with his late wife, Myrna I. Lewis, of “Love and Sex After Sixty.”

“Human relationships are important to the very end,” said Dr. Butler, who was not involved in the study.

The researchers, at the University of Chicago and the University of Toronto, contacted by letter a representative group of 3,005 older adults across the country. Trained interviewers then conducted a two-hour face-to-face session with each of these men and women, asking about their sexual activities as well as their physical and social health and other aspects of their lives.

The study, financed partly by the National Institutes of Health, found that 84 percent of men from 57 to 64 reported having had some sexual contact with another person in the last year, compared with 62 percent of women in the same age group. Those figures dwindled to 38 percent and 17 percent, respectively, in people 75 and older.