Using public information from birth certificates and prison records from Florida, we adopt a reduced-form approach to estimate the effect of relative age on the probability of incarceration in adulthood (until age 30-40). We use a Regression Discontinuity Design around the cutoff date for Kindergarten enrollment (Sept. 1). We find strong evidence of relative-age effects among black males convicted of drug-trafficking offenses. For that group, being born after Sept. decreases the probability of incarceration by roughly 12-20%. We find weak or no relative-age effects for other groups and types of offenses.