Researchers at the University of Kentucky followed 32 male baseball players between the ages of 13 and 21 for six years to study changes in the shoulder's range of motion, strength and growth plates. These were their conclusions. 

Throwing isn't necessarily bad.

Repeated pitching causes changes in the upper-arm bone and soft tissues in the shoulders of young baseball players, but these types of changes generally help protect players from injury and may help athletes develop better throwing velocity. Throwing is fine as long as it is done in moderation and by players and parents using common sense.

Young players need a break from playing.

Pitching too much or playing year-round can cause shoulder changes that go beyond normal adaptation and can lead to pain and even growth- plate injuries.

Nearly all young pitchers experience changes in their growth plate.

XC-rays showed changes in the growth plate in the throwing shoulder in almost all young pitchers, not only when there was a problem or pain as previously thought.