Shoulder adaptive changes in response to overhand throwing have been observed in adults, but the age of onset and progression of these adaptive changes have not been established. Two-hundred ninety-eight youth baseball players (8- to 28-year-olds) were studied to determine whether shoulder range of motion and laxity differences between the dominant and non-dominant shoulders exist between different age groups. The subjects were separated into 3 different age groups of 100 8- to 12-year-olds (Group 1), 100 13- to-14 year-olds (Group 2), and 98 15- to 28-year-olds (Group 3). For dominant shoulder external rotation with the humerus in abduction, all groups were significantly different from each other, with Group 2 having the greatest range and Group 1 having the smallest range (P < .05). When comparing dominant shoulder internal rotation in abduction among different groups, Group 3 and Group 2 motion was significantly less than that for Group 1 (P < .05.) When comparing dominant to non-dominant shoulder motion within each group, a significant increase in dominant shoulder external rotation in abduction was found in all 3 age groups (P < .05). Comparison of the differences in external rotation in abduction between the dominant and non-dominant shoulders demonstrated an increase with increasing age, Group 1 (1.5 +/- 6.8 degrees), Group 2 (9.6 +/- 15.3 degrees), and Group 3 (15.0 +/- 11.2 degrees; P < .05). Comparison of differences in internal rotation in abduction between dominant and non-dominant shoulders demonstrated a decrease with increasing age, Group 1 (4.6 +/- 8.2 degrees), Group 2 (8.4 +/- 14.5 degrees), and Group 3 (15.5 +/- 11.7 degrees; P < .05). For shoulder laxity, Groups 2 and 3 had significantly more inferior shoulder laxity when compared to Group 1. In summary, our results indicate that shoulder range of motion and laxity of youth baseball players are caused by adaptive changes that manifest during adolescence.