OBJECTIVES: Adolescents often display heterogenous trajectories of alcohol use. Initiation and escalation of drinking may be important predictors of later harms, including alcohol use disorder (AUD). Previous conceptualizations of these trajectories lacked adjustment for known confounders of adolescent drinking, which we aimed to address by modeling dynamic changes in drinking throughout adolescence while adjusting for covariates. METHODS: Survey data from a longitudinal cohort of Australian adolescents (n = 1813) were used to model latent class alcohol use trajectories over 5 annual follow-ups (mean age = 13.9 until 17.8 years). Regression models were used to determine whether child, parent, and peer factors at baseline (mean age = 12.9 years) predicted trajectory membership and whether trajectories predicted self-reported symptoms of AUD at the final follow-up (mean age = 18.8 years). RESULTS: We identified 4 classes: abstaining (n = 352); late-onset moderate drinking (n = 503); early-onset moderate drinking (n = 663); and early-onset heavy drinking (n = 295). Having more alcohol-specific household rules reduced risk of early-onset heavy drinking compared with late-onset moderate drinking (relative risk ratio: 0.31; 99.5% confidence interval [CI]: 0.11–0.83), whereas having more substance-using peers increased this risk (relative risk ratio: 3.43; 99.5% CI: 2.10–5.62). Early-onset heavy drinking increased odds of meeting criteria for AUD in early adulthood (odds ratio: 7.68; 99.5% CI: 2.41–24.47). CONCLUSIONS: Our study provides evidence that parenting factors and peer influences in early adolescence should be considered to reduce risk of later alcohol-related harm. Early initiation and heavy alcohol use throughout adolescence are associated with increased risk of alcohol-related harm compared with recommended maximum levels of consumption (late-onset, moderate drinking). Copyright © 2020 by the American Academy of Pediatrics.