Answer:
Teens should have specific goals towards physical activity, like train to high intense performance, which will improve their physical health, and their vigorousness, as youth, they must be strong people.
Also, some goals must be towards healthy habits, which it's probably the most important thing for teen agers, because at that age, people don't think in the future, and they don't look for any improvement in their life, they just don't care. So, by developing good healthy habits towards physical activity, they will get discipline, which is need during teen ages. Also, when they age, they will need those habits to maintain a good physical health.