Building on recent research examining athlete burnout trajectories, this study implemented the developmental model of sport participation to compare emotional and physical exhaustion, reduced sense of accomplishment, and sport devaluation between age groups (specializing [aged 13–15 years] vs. investment [aged 16–18 years]) and gender (boys vs. girls) among U.S. high school athletes. Participants were 367 high school athletes (M = 15.53; 212 males; 186 specializing) across various individual and team sports who completed a survey assessing their demographic information, sport backgrounds, and burnout perceptions. A 2 × 2 multivariate analysis of covariance, controlling for training hours, showed greater emotional and physical exhaustion and sport devaluation in the investment than the specializing group, but no developmental differences in reduced sense of accomplishment. Contrary to our hypothesis, no gender or interaction effects were found. Findings inform interventions and future research that address the role of developmental stages and gender in athlete burnout.