But for Liz, who is the single mother of a 24-year-old daughter, the sex drive has waned as she has aged.
"It just gets a lot harder," she said. "I look in the mirror, my mindset is sometimes 30 or 40, but then I take a realistic look at myself and my appearance. It's scary. And someone would want to jump in bed with me? For what reason? And even if you are doing it, it's just not quite the same."
Men and women responded differently on the survey: 21 percent of men admitted to sex outside marriage, compared to 11 percent of women. Men are also five times more likely to say they think of sex at least once a day and three times as likely to masturbate at least once a week.
But only 12 percent of the sexually active single men and only one-third of all women said they used condoms.
Previous studies have shown a rise in sexually transmitted diseases among the elderly, especially in nursing homes.
Older people are at an increasing risk for HIV/AIDS, according to the Centers for Disease Control and Prevention.
About 19 percent of all those with the disease are 50 and older, probably because they are not tested on a regular basis, the CDC says.
"They are simply not using condoms and it's dangerous," Schwartz said. "While the sexual transmission rate is low in this population, it's not absent and negligible."
Ten percent of the male respondents took medication to improve sexual functioning, and 23 percent reported being diagnosed for erectile dysfunction or impotence, according to the AARP survey.
Another big change is Americans are more approving of having sex before marriage.
Other studies in both the United States and abroad show that as men and women live longer, the number of people who are sexually active after age 50 has steadily risen since the 1970s.
A 2007 study in the New England Journal of Medicine reported that 73 percent of people in the 57 to 64 age group and 53 percent of those 65 to 74 reported having had sex with a partner in the previous year. Among 75- to 85-year-olds, about 36 percent were still sexually active.
And some married baby boomers such as Cynthia Lee, a 60-year-old English teacher from Maine, say financial woes can sometimes "bring you closer."
"When the economy takes a downturn and you have a good relationship, you gravitate more to sex if you don't have any money," she said.
Many people in middle-age and the older years are also turning to online-dating services.
Gina, a 56-year-old massage therapist from San Francisco with two children in their 20s, found a new lease on life through Match.com.
She discovered in 2008 that her husband of 27 years had fallen in love with his mistress.
"It hit me like a two-by-four," said Gina, who asked that her last name not be used. "He was the only man I had been with. I had never cheated on him. I was a devoted wife and believed in my vows and loved him."
When he asked for a divorce last year, her first words were, "You are going to do this in this economy?"
Skeptical, she went online and wrote a profile. Soon she met a wonderful man who "opened the door for me sexually," Gina said. "He kind of jump-started the engine.
"I was so scared but it was great," she said. "I am still alive. It was like being a virgin at 56."
Though they are still friends, the relationship didn't work out. But Gina went back online and met another man, this one a keeper, she said.
"I was always sexual and I liked sex and loved being close, but I had just lost it so long ago," she said. "I kissed a lot of toads before I found my prince.
"A lot of my friends who are divorced sit on the couch with their animals. But there's someone out there for everyone. You can have a whole new life again.
"Have the right attitude, don't be afraid," she said. "It really does work."