Ponce de Leon never found the fountain of youth, but the exercise he got during his search might have kept him young anyway. Exercise is the closest thing we have to a youth elixir. But schedules, commitments, and our own bodies change our fitness needs over time. Here are some tips to help you stay fit at any age:

Form lifelong habits: When you're in your 20s, your fitness goals might focus on looking good, but this is the time to look ahead and make exercise a lifelong habit. Find a routine that enhances strength, endurance and flexibility. Remember, your workout can do more for you than get you into your jeans; it can also help you lift your children, have more energy for work, and stave off age-related diseases.

Rev up your metabolism: As you age, your metabolism starts to slow down. That's why it's much harder to burn off those french fries in your 40s and 50s than it was in your 20s. Counteract this by mixing more intense bouts of exercise with rest periods to burn off more calories.

Focus on strength: Strength levels naturally start to decline after age 30. You can stay strong by adding a strength training component, such as weight training, to your exercise routine. This can also fend off age-related aches such as lower back pain.

Break it up: Fitness experts recommend at least 45 minutes of regular exercise, at least three times a week. If this seems difficult to stick to, remember that you don't have to do it all at once. Research shows that three 15-minute sessions per day can be as effective as one long workout.

A special note to Baby Boomers: The fastest growing segment of the gym-going population is over 50. Fitness experts suggest the 50+ crowd check with their doctors before starting a new exercise routine. Exercise is a great way to manage age-related illness, such as heart disease, diabetes and osteoporosis, so be sure your workout includes strength as well as cardiovascular training.