The aim of this study was to examine whether early and sustained organized youth sport during childhood and adolescence predicts the frequency of leisure-time physical activity (PA) at age 23 years. A 10-year longitudinal study of 630 adolescents was conducted. Data were collected from these participants eight times from the ages of 13 to 23 years and were analyzed by analysis of variance and regression. There was a high degree of consistency in participation in organized youth sport in terms of reporting to be a member of a sports club, especially from age 13 to 16 years. The correlation coefficients tended to be higher among males and decreased over time both in females and males. The age at becoming a member in organized sport and duration of participation in organized youth sports during adolescence predicted 9% of the variance of young adult PA. The correlations were higher in males than in females, but these differences were not significant. Organized youth sports during childhood and adolescence was positively related to frequency of leisure-time PA in young adulthood. Joining organized youth sports at an early age and continuing through adolescence appear to increase the likelihood for a physically active lifestyle in young adulthood.