The first purpose of this study was to longitudinally examine achievement goals in youth sport. The second purpose was to examine the within-person effects of perceived motivational climates by coaches, peers, and parents on achievement goal orientation. Participants were 78 young student-athletes, representing a variety of sports. The student-athletes completed a multi-section questionnaire, six times over a three-year period, assessing the study variables. Multilevel modelling analysis revealed that both task orientation and ego orientation decreased for this age group over the three-year period. Furthermore, perceived task-involving peer climate was significantly and positively related to task orientation, and perceived ego-involving coach climate was significantly and positively related to ego orientation. The results from this study provides insights regarding developmental changes in achievement goals and the importance of certain social agents in that specific developmental stage.