Parental permission to use alcohol is common in adolescence, and many parents believe it to be an effective harm reduction strategy because it provides an opportunity to supervise drinking. Contrary to this belief, prior research has consistently linked parental provision of alcohol and permission to drink to increases in future alcohol-related harms. Whether the age of onset of parental permission to use alcohol influences these outcomes is poorly understood. This study is the first to investigate the impact of age of onset of parental permission to use alcohol on later drinking outcomes, utilizing a longitudinal US community sample of adolescents (n = 387). The analysis included nine annual waves of data and accounted for risk and protective factors at the individual, peer, and family levels. Consistent with prior research, a robust relationship was found between parental permission to use alcohol during adolescence and increased alcohol use frequency and quantity, alcohol use disorder symptoms, and alcohol-related harms in young adulthood. Age of onset of parental permission was not associated with later alcohol use outcomes, suggesting a uniform risk effect of parental permission to drink. Public health messaging to parents should seek to correct perceptions of supervised alcohol use as a harm reduction strategy and emphasize the harm of parental permission to use alcohol, regardless of age.