Over 80% of adolescents worldwide are insufficiently active, posing massive public health and economic challenges. Declining physical activity (PA) and sex differences in PA consistently accompany transitions from childhood to adulthood in post-industrialized populations and are attributed to psychosocial and environmental factors. An overarching evolutionary theoretical framework and data from pre-industrialized populations are lacking. In this cross-sectional study we test a hypothesis from life history theory, that adolescent PA reductions reflect an evolved strategy to conserve energy, given the increasing sex-specific energetic demands for growth and reproductive maturation. Detailed measures of PA and pubertal maturation are assessed among Tsimane forager-farmers (age: 7-22 yrs.; 50% female, n=110). We find that 71% of Tsimane sampled meet World Health Organization PA guidelines (≥60 minutes/day of moderate-to-vigorous PA). Consistent with post-industrialized populations, we observe sex differences and inverse age-activity associations mediated by Tanner stage. Physical inactivity in adolescence is distinct from other health risk behaviors and also not merely resulting from obesogenic environments.