You've had a driver's license for years—and despite the late nights and pizza breakfasts of college, you couldn't be healthier. Let's face it, you're at your peak in your 20s. "It's all downhill from there," jokes Jordan Metzl, M.D., sports medicine physician at the Hospital for Special Surgery in New York City. The strategy for facing the uphill battle? "Get eight hours of sleep at night, exercise regularly, eat a well-balanced diet, and try not to let stress get to you," advises Dr. Metzl. Follow the expert pointers below, and you might even get carded well into your…