It depends which way he is throwing the ball. If he is twisting his wrist he could hurt his elbow and end up seriously injured. Only adults should twist their wrist when throwing a curve ball. There is a way to throw a curve ball without hurting your arm though. What you do is you get your grip on the Baseball and throw the ball like a football. I throw my curves this way and they are pretty nasty.
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Before the age of 16, a boy's arms do not have enough calcium in them to keep them from deforming from the stresses of throwing a curve ball. To be on the safe side, as well as to give enough time prepare for either a college or pro team, the junior year of high school would probably be the earliest time to allow it.
It depends on your body type and strength, typically a 8 year old shouldn't try it but a 16 year old could.
different people say different things. i personally think only a few in one game at thirteen and throwing them regularly at the age of fifteen.