Studies have shown that the incidence of elbow pain in youth baseball players is between 20% and 30% for 8- to 12-year-olds, approximately 45% for 13- to 14-year-olds, and over 50% for high school, college, and professional athletes. Whether this is due to pitch counts being inadequate, not being enforced, not the cause, or to an increase in diagnosis, is hard to determine. A cross-sectional study of 754 pitchers, 9-18 years old, reported that 45% of the pitchers had no pitch count in place and over 13% pitched more than the recommended 8 months in a 1-year period.
Source: American Journal of Sports Medicine, January 2016