In many developed countries, children now begin their formal schooling at an older age. However, a growing body of empirical studies provides little evidence that such schooling delays improve educational and economic outcomes. This study presents new evidence on whether school starting age influences student outcomes by relying on linked Danish survey and register data that include several distinct, widely used, and validated measures of mental health that are reported out-of-school among similarly aged children. We estimate the causal effects of delayed school enrollment using a "fuzzy" regression-discontinuity design based on exact dates of birth and the fact that, in Denmark, children typically enroll in school during the calendar year in which they turn six. We find that a one-year delay in the start of school dramatically reduces inattention/hyperactivity at age 7 (effect size = -0.7), a measure of self regulation with strong negative links to student achievement. We also find that this large and targeted effect persists at age 11. However, the estimated effects of school starting age on other mental-health constructs, which have weaker links to subsequent student achievement, are smaller and less persistent.