Because they are growing and they stay active.
They burn energy faster! Anyone over 19 has high metabolism because their nickname is "Thumper" ;)
Most of them are very active. That is why I think they have a high motabilisum. Peace
Because they are still growing and their bodies are still maturing along with their hormones, crouchieness, and everything else that goes along being a teenager.
I have been fortunate, and have had this bliss with a high motabolism, but now it seems to be running out. Lol Youth is the reason, and being active helps it along.