I think (again...think) that what is worrisome to most about young players throwing a curve ball are the ones who are allowing them to be thrown before the growth plates come together. Like Dr. Andrews article says, usually by the time they can begin shaving is a pretty safe time for them to start learning.
I remember when Younggunson was playing LL and there was this other team in the league that had a lefty who was nothing but lights out. Not only did he throw hard but he had a filthy curveball. Of course, being that we had only being playing ball for a couple of years we were just as mesmerized by his pitching as everyone else.
Fastfoward to his Senior year in HS when we faced him. Son asked if he was pitching against us that night and he said that he only plays 1B now. His arm/elbow killed him to pitch now and gave it up.
That was the last year he played ball again.
True story. It's sad because I liked the kid and his family.
YGD