Nelly is the oldest prostitute that her Manhattan-based madam, Gina, has. Gina specializes in mature women, and Nelly, in the marketplace of the troisième age, is the matriarch. She's Brazilian, her thin lips are painted brown, a tiny mezuzah hangs around her neck. I want to call her grandmotherly, but that would ruin everything. She's been working for Gina for nineteen years and escorting for forty. She tells me this will be her last year. "I had lots of friends," she says, "but I don't keep looking for them. I don't put out ads. And so they disappear year by year."

In her red sweater, her breasts are impressive. When I ask to see them, she hitches up her bra and out they roll: firm, effulgent, with a deep blue vein running up to the areola. They are entirely without wrinkles, untouched by plastic surgery. I run my fingers across them and cup them appreciatively. "Young men, they want to be between them," she says proudly. "American men love breasts."

I've come to meet Nelly because, according to the world of sex research, she shouldn't exist. Male desire isn't supposed to attach itself to her. The main studies of American male sexuality — the Kinsey Report, Masters & Johnson, the University of Chicago Sex Survey — look at nearly every other key of erotic attraction but the one lying across from me. We have stats on how many men have had sex with animals (8 percent in the total population, 17 percent among rural boys) but no idea how many have lusted after their English teachers, their aunts or a classy, bossy "governess," a role that Nelly considers her forte. She maintains twenty-five regular customers a month, not despite her age but specifically because of it. When I ask her if she knows other older prostitutes, she says she knows a lot, some even older than her. The fact of Nelly, and others like her, suggests a dynamic of attraction overlooked by social sciences.

More than half the guys Nelly sees are in their twenties, thirties and forties. They are her preferred clientele.

They're more gentle, they listen. Men her own age have precise instructions for her to follow.

They're slow, she says, and "they only talk about diseases, how their friends have died, and how they have physical problems." Younger guys want romance. They want to surrender control. "I make the decisions, I'm the boss," she says, although domination doesn't figure into Nelly's work. It makes Nelly depressed. Usually, she doesn't even have sex. One guy in his thirties, who only wants women sixty and up, arranges to have her bestow gentle kisses on his head. Others only want to grope her. Though she enjoys their company, Nelly has never fallen in love with a younger client. "I feel something for them in my heart — I care," she says. Her longest, most faithful companion is Oscar, the Yorkshire terrier that rides shotgun in her purse.

Prostitutes have long been the substitute teachers in sex ed, and Nelly's clients come to her to be trained. "They want to do a lot of things they have read about, but they don't know how. So I have to teach them. I had this one guy who was twenty-five who wanted to know how to do . . . you know . . . oral sex, and I taught him." It's strange to see Nelly turn suddenly shy at the details, but it is one of the sexiest things about her. Nelly's clients have to overcome a double shyness: not only are they inexperienced, but they have sought out an older woman, an attraction that's not socially endorsed. They feel a social censure for finding women like Nelly sexy. This is where Nelly comes in. She's not intimidating, nor judgmental about skill, lest she be judged by her age.

Gina, Nelly's frosted-blond, full-figured boss, thinks that the desire of younger men for older women is a kind of narcissism. When the woman is so much older, she says, the younger man's beauty necessarily becomes the center of attention. "I ask them, 'Why do you like older women?' They say it's because they're more experienced," she says. "But in the end, they all want the same cliché. The women compliment them. They always say, 'You have such a beautiful young body.'"