Aside from their meteoric careers, comedian Tina Fey, child star Brooke Shields and Coldplay front man Chris Martin appear to be united by something else: protracted virginal phases.

This week, when David Letterman asked Ms. Fey about losing her virginity at 24, the comedian said she “couldn't give it away” because she was “homely.”

Mr. Letterman had broached the subject by reading aloud a list of other notable late bloomers, including Victoria's Secret model Adriana Lima, who gave it up at 27, Mr. Martin, who lost it at 22, and Ms. Shields, also virginal until 22.

The list was surprising, in part because today's celebrity virgins cue visions of the bright-eyed, bushy-tailed Jonas brothers, or, on the other end of the spectrum, the massive-mammaried Natalie Dylan, who has tried and so far failed to sell her virginity to the highest online bidder.

Unlike the religiously – or, in Ms. Dylan's case, the sensationally – abstinent, the “late bloomer” remains a far more socially nebulous creature.

While some wait for the “right one,” others say they have bigger things on their minds – doctorate degrees, for instance.

Some blame an ugly-duckling phase, and others look forward to the pent-up sexual energy they'll unleash on their first partner.

What binds them together is the curiosity and derision they often face from the sexually active. Some people regard adult virgins as asocial freaks. Others say they are excessively cautious, barricading themselves against the prospect of an ill-chosen first lover in perpetuity.

For those who admit their sex-free status, the pressure to “git 'er done” ends up simmering beneath most social exchanges. And the stigma can unnerve even the most self-assured virgin, experts say.

Late bloomers are fascinating because they are “atypical,” said Terry Humphreys, an associate psychology professor at Trent University whose primary research interests include first-time sexual experience.

Prof. Humphreys said that if people aren't abstinent because of religion, “then we say, ‘What's the reason, then?'”

Although the median age for first intercourse in Canada is 17, Prof. Humphreys said many do the deed well before – and well after – that age.

His survey of first-year students at Trent, Wilfrid Laurier and Guelph universities found that 25 per cent reported being virgins.

“It's not for lack of sexual desire,” Prof. Humphreys said. “They're waiting for a good reason to come along. A lot of people say they're just not ready.”

But once virginity persists into adulthood, it can start to feel like a ball and chain, Prof. Humphreys said.

“The longer it goes on and they haven't transitioned, the more likely they are to switch to a stigma mentality that says, ‘Why hasn't this happened for me yet when it's happened for everyone else?'”

In online forums such as “I Am a Late Bloomer,” “I'm Still a Virgin and Don't Want to Be” and “48-year-old Virgin, Why Do Women Hate Me,” aging virgins share their pained predicaments. (The forums appear on ExperienceProject.com, a website that lets users post and comment on each other's stories.)

“I am not waiting to have sex until I get married; I just want to be with a partner who understands me,” a woman named Pau writes in her story, “24-year-old virgin.”

Although she had the opportunity at age 16, Pau decided to wait.

“I've always been a late bloomer, so I thought this as everything else would work itself out through time; without forcing or pushing.”

But the pressure began to mount when she turned 22. Since her chastity had already “scared away two guys,” Pau now lies about it.

“I often feel like I am THE ONLY virgin in this world!” she exclaims.

The adult virgins who visit Toronto-based sexologist Jessica O'Reilly often ask, “Am I normal?”

“We're prescribed this cultural norm of sexuality,” Ms. O'Reilly said.

“God forbid you have sex at 16, that's too young. But if you haven't had sex at 20, well, that's too old.”

Experts say these rigid prescriptions are harmful.

“We have an oversexualized culture that considers sex a marker. In one way or another we are judged by that,” said Jessica Valenti, author of The Purity Myth: How America's Obsession with Virginity Is Hurting Young Women , published in April.

Even sex idols who waited wished they'd jumped in earlier.

Ms. Shields said her persistent virginal phase was her “biggest health regret.” In a May interview with Health magazine, the model said her early exposure made her grow a “protective 20 pounds” in college.

According to an article in The Guardian, Chris Martin was still a virgin as Coldplay recorded its first album. The singer has disparaged himself as a “self-conscious uncool fool.”

Late-blooming men are particularly stigmatized, Ms. Valenti said.

“Virginity in males is considered a bad thing. For a woman, she can still exist under the framework of being a good girl.”

For both men and women, a number of predictors exist for virginity into adulthood (past age 20, that is). Prof. Humphreys said they include being religious, having a higher body mass index and maturing later than your peer group.

But despite awkward starts, late blooming can have its advantages in the bedroom later on, Ms. O'Reilly said.

“Choosing to postpone partnered sex until a later age can be a healthy decision, as long as it's motivated by personal choice and not negative sexual repression,” she said. “There can be a host of benefits around masturbation, learning about your own sexual responses, your own likes and dislikes. With age comes an improved ability to communicate and express yourself.”