Ask a senior citizen when sexual desire comes crashing to a halt and you might be surprised to hear the response: "I'll let you know when I get there."

With the number of Americans over 65 projected to be 53 million in 20 years, senior sex is no oxymoron. And while it may be stunning for some to think of Gram or Gramps getting it on, the elderly say they've got more flow than ebb. 

Tony Aiello, 61, and his wife Anne, 60, "take advantage of society's prejudice" by playing sexy games in public places like restaurants, he writes in Still Doing It: Women and Men over 60 Write About Their Sexuality, a new anthology from San Francisco's Down There Press. 

"No one is aware that we two old and gray codgers are engaged in sex play" using a kinky sex toy complete with a remote control, he writes. "We look around at our younger fellow diners and wonder if they'll ever be this playful when they, too, are old and gray." 

Thank You, Viagra 

What's behind this senior sexual Renaissance? Medical science — Viagra, longer life expectancies — plus changing social attitudes equals more seniors who are demonstrating that sex isn't the sole provenance of the young. 

In one study at the University of Maryland, a group of student nurses were shown pictures of a 68-year-old man and woman. The study participants rated the sexually active seniors more favorably — a surprise to investigator Shirley Damrosch. 

"Our puritanical society frowns on older people being sexually active after a certain period, or at least it used to," she says. "Of course these are nurses — I don't know how widespread this enlightened attitude is in the general population." 

Where do the prejudices against older people having sex come from? 

"From an evolutionary perspective, sex is for the young," says Dr. Walter Bortz of Stanford University, a former president of the American Geriatric Society. So any sex that doesn't result in children, including people past their reproductive years, is taboo. 

But that viewpoint is only true if reproduction is the sole goal of getting it on, Bortz adds. And if sex is seen in a broader context, that "sex is for the maintenance of intimacy, sensuality and fullness of personal expression, you have to sustain it," he says. 

'I'm Getting Randier and Randier' 

There are signs senior sex is slipping into popular culture. 

One faux television ad for Viagra posted on the popular Web site Adcritic.com has an attractive young woman trying to piece together her night after she wakes up in a strange bed. She rolls over to find a septuagenarian, and the ad cuts to the tagline: "It's all about the Viagra — are you on the Pill?" [Click here to see the ad; Quicktime required] 

Of course, the seniors who contributed to Still Doing It don't much care what everybody else thinks. 

"I'm 60, and I can drive a man crazy with desire by how I look and act," says Anne Aielo, in Still Doing It. "I'm supposed to be way past any sexual desire, yet I'm getting randier and randier as I age." 

Denture On or Off? 

Two-thirds of people over 60 are "extremely or very satisfied with their sexual relationships," according to a survey by Modern Maturity, but sex after 60 is not without problems. 

"Being older lovers has its consternations, as well as its comforting hilarity," writes Annie Maine. "The partials [dentures] — what do I do with them when my lover's staying overnight? My gums beg for relief, but I worry that romance will be threatened if he should wake up to a partially toothless wonder." 

For men, sexual problems are almost always erection-related. Virtually all men find it more difficult to get erections as they age. Five percent of men at 40, 25 percent of men at age 65 and 55 percent of men at age 75 have problems with their erections. 

"Mine are not as firm as they were," writes Bruce Eastwood in the chapter "Seasonal Changes." "Occasionally I will lose one in the middle of intercourse ... these problems are to be expected. The good news is that these occur infrequently and little diminish the intensity of [his partner] Mara's and my enjoyment of each other." 

As for women, post-menopausal women often have to deal with vaginal dryness and a lowered libido, which can be counteracted with hormone treatment and lubricant. 

These difficulties don't mean the loving isn't as good, but expectations of what constitutes mind-blowing sex need to be adjusted. 

"I don't expect my partners to have a full-blown erection every night," says Maine in her piece. "Men are so relieved when you tell them that. It now takes both of us longer. Sometimes neither of us does before we fall into blissful sleep."