Although the rate of alcohol use among adolescents has declined, it remains their drug of choice. Parent and peer alcohol use are powerful risk factors for youth alcohol use. However, questions remain about how these factors influence underage drinking. The present study investigates the relationship between exposure to parent or peer alcohol use and two stages of adolescent drinking—onset and escalation—overall and at five age points during adolescence. Participants were 9348 adolescents in Waves I (WI) and II (WII) of the National Longitudinal Study of Adolescent Health, whose parents completed interviews at WI, and who identified themselves as either non-drinkers or experimental drinkers at WI. Reports of WII alcohol use were used to measure onset among WI non-drinkers and escalation among WI experimenters. Risk ratios were calculated to assess the overall impact of exposure to parent or peer alcohol use on onset and escalation, and at five age points (i.e., ≤ 13, 14, 15, 16, and ≥ 17). Findings show that exposure to either parent or friend alcohol use increased the risk of onset and escalation. Age-based analyses reveal a more nuanced relationship, showing variability in the nature and strength of influence by stage of drinking and by age. This study highlights the relevance of both parent and peer modeling on youth drinking throughout adolescence. Implications in advancing prevention and treatment include parental education about the impact of their own behaviors and the importance of monitoring teens’ friendships.