I practise Tae Kwon Do since i was 7 or 8 (im 19). I think its a good sport, but also requires a good physical preparation and there is always the chance of having some kind of lesion (this sport isnt good for the back, and also the knees may suffer, for example my knees are very bad now).
If you wanna practise just to relax and keep fit, Tae Kwon Do is great, but i also think that other martial arts like Karate, Kung Fu or Jiu Jitsu will be OK too.
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"Let my spells do the dirty work" Ertai, High Sorceror of the Holy Flame