"This statistic had become almost a mantra among dermatologists," says Katie Rodan, MD, adjunct assistant professor of dermatology at Stanford University School of Medicine. "But now we're very happy to be proved wrong." A recent study revealed that we actually get only 23 percent of our total UV exposure by age 18. At 40 it reaches about 46 percent; by 59, 74 percent. "No one can say anymore, 'I've already blown it,'" says Rodan. "You can still prevent sun-induced aging by getting serious about sunscreen."