This paper estimates the causal impacts of delayed primary school enrollment on children's educational outcomes in rural China. Instrumental variable estimates exploiting the discontinuity in children's enrollment age around the enrollment cut-off date indicate that a one-year delay in school enrollment increases the incidence of first-grade retention by approximately 10 percentage points for boys and reduces the probabilities of middle school enrollment by 6 percentage points for both boys and girls. These results suggest that delayed enrollment, even if it may be an optimal choice made by poor parents in response to financial constraints, is likely to be harmful to children's educational development in rural China.