There is no doubt that someone who tries to throw a curve or pitch at any early age before he's developed, before his hand is big enough to grip the ball correctly, will damage his arm. ↗
An actor would be foolish to do something that might hold up the picture, or more importantly incapacitate him. If an actor does do a stunt he needs to make sure a stunt man stands by to see that it's done correctly. ↗