Call up a mental picture, for a moment, of Dina Lohan. In your mind's eye, the 45-year-old stage mom is probably wearing a low-cut top and a denim miniskirt. Maybe a pair of UGGs. In other words, an outfit a lot like one her 21-year-old daughter would wear. If you're reading this story and you're a 35-plus-year-old wearing a faux-vintage Urban Outfitters T-shirt and plaid Vans, you and Dina Lohan have a lot in common. Only, not having a honey-colored spray-on tan and highlights, you don't look as good.

In addition to the midriff-baring mom, you're inadvertently aligning yourself with another type you probably scorn: the Midlife Crisis Man. Being 38 and wearing a retro Sea World tee under a hoodie to Sunday brunch is essentially the same as being 48 and wearing a leather bomber and dog tags. You and Midlife Crisis Man—see Anthony Bourdain (dressing like a punk might be his thing, but the man is over 50) and Harrison Ford (the dad jeans don't cancel out the fear-of-mortality earring)—each might as well be wearing a big ol' baseball hat that says I'M AFRAID OF GETTING OLD.

"I don't get it," says Tim Gunn, Project Runway mentor and chief creative officer at Liz Claiborne. "I think men look older when they try to dress young. You stop and look at them, because there's something incongruous about it. And then you realize—wait, this person is way too old to be wearing those clothes."

"So many people have a distorted view of themselves," Gunn, who's 54, adds. "I remember once a few years ago seeing my reflection in a department-store mirror, and for a moment I thought, What's my father doing here?"

What motivates some men to cling to the vestments of their youth like a 4-year-old to the last pair of SpongeBob SquarePants pajamas in Toys "R" Us no doubt varies. But it can usually be categorized in one of two ways: deliberate (you don't get your ear pierced at 52 by accident) or unconscious (If I don't remember that 35th birthday, then it didn't really happen, right?). And it manifests itself in ways both subtle and heartbreakingly obvious.

"I've found recently that older guys are wearing clothes that are far, far too small for them," says British men's designer Oliver Spencer. "There's nothing worse than an [old] guy who's very fashion-conscious but not in shape wearing small shirts."

"A 45- or 50-year-old guy shouldn't be wearing ripped jeans or leather jackets," Spencer continues. "They shouldn't even be wearing jeans that are all washed out—those are for kids."

But what exactly is the turning point? How do you know when it's time to shed the uniform of your twenties for good?