The present research examined how the within‐person association between sexual initiation and internalizing symptoms decays over time, using data with annual measurement occasions across adolescence (N = 1,789) and statistical models of within‐person change. Sexual initiation was associated with increased levels of internalizing symptoms for early‐initiating girls (ninth grade, approximately age 15), but not for on‐time‐initiating girls or for boys. The association between girls' early sexual initiation and internalizing symptoms declined precipitously over time. Indeed, 1 year after sexual debut, early‐initiating girls were similar to on‐time or noninitiating girls on internalizing symptoms, suggesting early sexual initiation does not produce lasting detriments to girls' mental health. Findings inform how researchers perceive sexual initiation, both as a developmental milestone and as a prevention target.