"The most prevalent and forgivable reason men lie about their age is professional insecurity," says K. Cooper Ray, the writer behind the etiquette blog SocialPrimer: Manners, Conversation, Style & Handling Your Liquor. "You hit 40, so you fudge that you're 39, because you see all the new young Turks under you with their fabulous careers."

Dora, a journalist in her thirties who covers (and dates within) San Francisco's silicon fishbowl, says she can't help but judge men based on their age. "When I meet a guy who's a tech genius in San Francisco and hear he's 30," she says, "I think, 'Eh, he can't be that good.'"

Being considered a failure for not having hit the peak of your career by age 30 can make even wunderkinder nervous enough to lie. Tom Anderson, a cofounder of MySpace, was lambasted last year for stating online that he was 32—in fact, he's in his late thirties. Networking sites, of course, are where the most flagrant age deflation among men thrives. "This guy I know who is only 35 says he is 24 on his MySpace profile," says one female devotee of the site. "Obviously he did it to get dates." But pandering to youth culture is par for the course, even when you're not trying to get laid. Doug, a married guy from Arizona who's in his mid-thirties and plays guitar and sings in a band, says, "When I say I'm 25 in my profile, I get 10,000 hits. When I put it back to 34, I get like 212."

"It's always interesting to see how age is listed on profiles on Facebook," says David, a designer at a major Internet company who, though he's not yet 30, is keenly aware of age sensitivity among his male colleagues. "It's always 'May 3,' not 'May 3, 1972.' No one lists years unless they are in their early twenties."

Age manipulation is the kind of lying that takes finesse and delicacy. Women have perfected it, having endured decades of cultural pressure—and propaganda from a billion-dollar cosmetics industry—to stay 29 forever. They are discreet, assuming that the people they're misrepresenting themselves to are in on the deception: Here's the deal—if I look good, you don't ask how old I am. Online-dating researchers at the University of California-Berkeley have found that men, conversely, lie brazenly about their ages because they think they can get away with it, using the same logic that brought the world comb-overs and two-inch lifts.

In the end, pretending to be 23 when you're 42 is just gateway behavior—a move toward joining that creepy club of guys in their late thirties and forties who look like walking testaments to the wonders of lat pulls, dermabrasion, and Cialis. You can spot them a mile away—wearing flip-flops, Abercrombie & Fitch shirts, and cargo pants, plumped and pumped up into some non-age. So if you're tempted to start airbrushing away a few birthdays, ask yourself the same question you'd ask those guys if it weren't impolite: Excuse me—whom exactly do you think you're fooling?