Tobacco, alcohol, and marijuana are among the most commonly used drugs during adolescence. Initiation of marijuana use typically peaks at age 15, with risk of initiation continuing throughout adolescence. The goal of the current study was to prospectively examine the influence of age of marijuana initiation on four outcomes: physical health, mental health, illicit drug use other than marijuana, and marijuana-use related consequences at age 18. We controlled for several important predictors of adolescent drug use and its associated consequences, including demographics, social bonding variables, personality variables, and recent use of marijuana. Baseline survey data were collected in 1984 at grade 7 and follow up surveys were conducted at grades 8, 9, 10, and 12 (N = 2079). This initiates-only sample was 47% female, 66% White, 11% African American, 13% Hispanic, 5% Asian, and 5% other race or ethnicity. Findings indicated that age of initiation predicted marijuana consequences and other illicit drug use after controlling for demographic, social, and behavioral factors. However, once frequency of recent marijuana use was included in the models, age of initiation was only associated with other illicit drug use. Both primary and secondary prevention are needed to curb marijuana use and its associated harms.