Just a comment on the curveball debate ... Over the years I've seen several current and former MLBers make absurd statements about throwing curves. It was obvious by their comments there was nothing scientific behind their statements. It was purely anecdotal combined with the assumption they knew something.
Do I know kids who toasted their arms throwing curves? Yes. I also know kids who toasted their arms throwing nothing but fastballs. It isn't the pitch. It's the ability to throw the pitch properly.
To me, the biggest issue with throwing a curve at young ages is the player learning how to throw it properly and a coach recognizing failing mechanics due to fatigue. Watch a very slow motion video of a fastball versus a curve. The motion of a fastball is much more violent than the curve. Watch the snap back after the release.
This is all I'm going to say. I've done the curveball debate to death.