A sizable literature examines the effect of transferring the most serious and persistent juvenile arrestees from the juvenile justice system to the adult justice system. Few studies, however, have tested whether processing juveniles in the adult system has a similar effect on those who are not eligible for transfer. Recent legislative changes to the exclusive jurisdiction of the juvenile justice system justify greater scholarly attention on this population. Using a regression discontinuity design, this study estimates the effects of the juvenile and adult justice systems on recidivism for non-transfer-eligible juvenile offenders arrested just weeks before and after the age of majority for drug distribution, a common felony charge. Our results suggest that processing these juveniles as adults slightly reduces the probability of recidivism by between 3 and 5 percent. Based on the rapid onset and limited change in size of these effects over the duration of a 4-year follow-up, and based on the concentration of the effects within a sub-group having the lowest risk of incarceration, we attribute this finding to a combination of enhanced deterrence and incapacitation in the adult justice system. These results suggest that processing juveniles in the adult system may not uniformly increase offending and may reduce offending in some circumstances. Implications for recent state-level changes to the jurisdiction of the juvenile justice system are discussed.