The Verdict: Need More Info
You are not old enough yet: the Foreign Legion takes candidates from 17. Come back to this when you turn 17, with both parents' written consent if you are under 18.
The upper limit is under 40, so there is no rush. Selection includes medical, physical and psychological tests, and fitness is the most common reason candidates are turned away.
In the meantime, what helps most is physical fitness, a clean record and a valid passport.