The effects of school starting age and relative age effects (RAEs) have generated much interest from parents, teachers, policymakers, and educational researchers. Our 10-year longitudinal study is based on a nationally representative (N = 4,983) prospective sample from the Longitudinal Study of Australian Children. The primary outcomes are results from the high-stake, Australia-wide National Assessment Program-Literacy and Numeracy tests in Years 3, 5, 7, and 9, controlling for demographic characteristics (gender, socioeconomic status, school type, and childhood cognition measured before the start of kindergarten). We evaluated how direct and mediated RAEs vary over the first 10 years of schooling for these longitudinal data. Results revealed significant RAEs in primary school years for both numeracy and literacy test outcomes. Effects were large in primary school years but declined in secondary school years. Although the direct effects of RAEs declined over time, there continued to be significant indirect effects over the whole 10-year period. RAEs in primary school had enduring effects that were mediated through the effects of earlier achievement. We juxtapose our results with previous RAE research on achievement and a range of other noncognitive outcomes where the RAEs are enduring into adolescence and even adulthood. We position our research within this broader research literature and discuss implications for educational policy, practice, theory, and future research.