PURPOSE: This study aimed to characterize longitudinal age trajectories across 5 yr in the prevalence of free-time and organized physical activity participation among US youth by sex, race, and parental education. METHODS: Study participants were a nationally representative sample of youth, 9-13 yr old in 2002, who participated in the Centers for Disease Control and Prevention's Youth Media Campaign Longitudinal Survey. Baseline data were collected in 2002. Attrition for the next 4 yr resulted in an overall response rate of 23% by 2006 (n = 1623). The survey collected information concerning respondents' frequency of participation in free-time and organized physical activities outside school. Organized activities were defined as activities involving a coach, instructor, or other leader. Orthogonal polynomial contrasts were used to test for linear and quadratic trends in respondents' participation free-time and organized physical activity sessions during the previous 7 d over ages 9-17. Pairwise t-tests were used to determine whether age-specific estimates of participation rates differed significantly by sex, race, and parental education level. RESULTS: Free-time physical activity participation prevalence declined linearly from ages 9 to 17 in both sexes but also demonstrated a quadratic trajectory in boys, peaking at age 13. Organized physical activity demonstrated a quadratic trajectory and declined most notably after age 14 in both sexes. Free-time physical activity participation was lower in girls compared with boys between ages 12 and 16 (difference range = 12-17 percentage points). Both non-white youth and those with less educated parents had lower organized physical activity participation at most ages (difference range = 15-29 percentage points). CONCLUSIONS: Free-time and organized physical activity exhibit different trajectories between ages 9 and 17 and are subject to dissimilar demographic level variation.