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 30s.

Cardiovascular capacity:  "Because your heart's a muscle, it should be trained to optimize its functionality—just as you'd train to get bigger biceps," says Dr. Metzl. But instead of biceps curls, you'll need to get your heart pumping. How? Walking, running, swimming, biking, any aerobic exercise that will get your heart rate into a training zone: 60 percent to 90 percent of your maximum heart rate, or the maximum beats per minute. Maximum heart rate is calculated by subtracting your age from 220. The American College of Sports Medicine recommends sustaining that level for 30 minutes, three to five times a week.

Muscular strength:  "If you're training hard enough and appropriately, your muscles are operating at their peak strength," says Jeanne Nichols, Ph.D., professor of exercise physiology at San Diego State University. But if you're not training efficiently, now's the time to start. "It's not too late," she says. Nichols advises that you follow the American College of Sports Medicine's (ACSM) guidelines, which recommend strength training two or three times a week. During those strength training sessions, the ACSM says, you should complete eight to 12 repetitions of eight to 10 exercises that work all of the major muscle groups (arms, shoulders, chest, abdomen, back, hips, and legs).

Flexibility: "Most people don't stretch at all," says Nichols. "But stretching is important in maintaining range of motion and mobility in the joints—which becomes especially important as we age." The ACSM advises stretching your major muscle groups at least two to three times each week, and Nichols recommends paying extra attention to areas that feel particularly tight, such as your neck and back. The National Strength and Conditioning Association recommends the following stretches to keep your neck and back feeling relaxed.

Neck:  Drop your head toward your left shoulder until you feel a bit of tension in the right side of your neck. Hold the position for 15 seconds and slowly bring your head back to the starting position. Then gently drop it down to the right and hold for 15 seconds. Repeat the stretch three times.

Drop your chin towards your chest until you feel light tension in the back of your neck. Hold the position for 15 seconds and repeat three times.