Typically the life of a 15 year old boy is the appropriate exercise for a 15 year old boy. However, in this country we tend to weigh young people down with tv/games, food lacking nutrients, and the problems of the adults around them.
From a purely yogic perspective the postures for a young person are dependent on the young person. But I can't think of anything I'd generally avoid. Most of your question can be answered with two words - proper nutrition!
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