Teenagers lead tremendously packed lives, as they divide their time between school, tuitions, co-curricular and extra-curricular activities. Given the limited time they have on hand to attend to their fitness needs, teenagers are more susceptible to obesity and weight gain owing to long hours of being hunched over school work and getting less activity.
Teenage fitness is important for many reasons. The first and most significant reason is that teenage years are the formative years in terms of health, laying the foundation for the future. Consequently, obese and overweight teenagers are on the path to a future beset with lifestyle diseases such as high blood pressure, cholesterol, heart problems, diabetes, hypertension, which in turn leads to unhealthy life.
The second reason why teenage fitness is important is because it reduces stress and encourages greater levels of concentration and productivity. By allowing time to incorporate exercise and fitness, teenagers are opening their minds and bodies to a state of near-low-stress functioning, which improves the extent of their retention capacity and their application of what they learn.
Eating healthy is very vital for teenagers since their formative years require the touch of nutrients to the greatest extent. A balanced diet is especially important because it gives them the healthy nutrition that fast food cannot comfortably offer. Teenagers need to set the ball rolling on health for the future by eating right and staying away from too much junk food and unhealthy eating habits.
Finally, teenagers gain amply from a sense of discipline and mental fitness by indulging in a good and healthy fitness regimen. By eating foods that are natural and cooked healthily, by exercising regularly, by sleeping on time and waking on time with a minimum of 6 to 8 hours of sleep within, and by choosing a more active lifestyle, teenagers create habits that they remember and carry on for years to come.

References:
http://timesofindia.indiatimes.com/life-style/health-fitness/fitness/Why-exercise-is-important-for-teenagers/articleshow/46968372.cms
http://exercise.about.com/cs/exercisehealth/a/teenagers.htm
http://teens.webmd.com/aerobic-exercise-for-teens

Picture Courtesy: peakwellness.com