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Children starting school older consistently exhibit better educational outcomes. In this paper, we underscore child development as a mechanism driving this effect. Using unique administrative data on health examinations, we study the causal effect of school starting age (SSA) on a child's probability of being diagnosed with special educational needs in early grades. Results show that children with higher SSA are less likely to develop behavioral problems and speech impediments, whereas learning disabilities, ADHD, and dyslexia/dyscalculia remain unaffected. Importantly, these effects only arise after primary school entry and are not due to preexisting health conditions. We also find that teachers tend to over‐refer relatively young children to special needs services, but this over‐referring behavior is not driving the results, which are based on psychologists' diagnoses. The SSA effect persists throughout compulsory schooling, resulting in higher test scores and better quality vocational training contracts. However, SSA does not affect employment, earnings, or disability insurance benefits at labor market entry.