Marnie Unruh | Aquafresh brand manager | GlaxoSmithKline

Most skeptics think that something will be age. Adult twins aren't as interesting as babies, toddlers, or teens, the thinking goes. "The biggest question is what's going to happen as they get older," notes Brochstein. "They're going to start college next year. How long can they hold onto the image and move their audiences? Do they just become frozen in time?"

The answer to the last question is: Why not? And here's a for-instance: GlaxoSmithKline Consumer Healthcare recently licensed the Mary-Kate and Ashley brand for Aquafresh toothpaste. The twins personally selected the Bubble Cool Flavor, proposed the tagline "It's How You Smile," and picked the photo that adorns the package, according to Marnie Unruh, Aquafresh brand manager. Glaxo is happy with the product's early performance, says Unruh, and isn't worried about the twins outgrowing the 'tween market it's directed toward. Why? "The photo that appears on the toothpaste [tube] is a picture of Mary-Kate and Ashley at 13," she says, noting that the girls are now 17. "This picture was chosen to be appealing to the toothpaste's age demographic," she adds. "We have no current plans to change the photo." This strategy isn't limited to toothpaste, either. When introducing the brand to foreign markets that didn't watch the girls grow up, they're also presented as much younger than they are now.

Nevertheless, Thorne scoffs at suggestions the girls won't appeal as women. "People are wrong," he says. As Mary-Kate and Ashley get older, he says, their appeal to young girls stays strong, while their core fans age right along with them. "Why would the appeal be limited?" he asks. Still, Wal-Mart's Clancy says the retailer tried a line of Mary-Kate and Ashley apparel aimed at older girls and was unhappy with the results. "We've tested it twice and won't be going forward with it in juniors," she says. "We think the appeal is really quite young."