This article reviews the growing literature on age at school entry and its effects over the life course. Age at school entry affects a broad range of outcomes, including education, labor-market performance, health, social relationships, and family formation. We synthesize the evidence using a conceptual framework that distinguishes four empirically intertwined components of age at school entry: starting age, age at outcome, relative age, and time in school. Within this framework, we highlight six key channels through which age at school entry operates. While the effects of age at school entry are often substantial and persistent, many studies estimate bundled impacts without isolating specific components or directly measuring underlying mechanisms. We explain how different research designs capture distinct combinations of these components. We also highlight how institutional heterogeneity and behavioral responses can complicate the interpretation of results. We conclude by outlining directions for future research and policy design.