Background: Alcohol and marijuana are the most commonly used psychoactive substances; however, the sequencing and relationship between age of first use and continued current problematic use among college-bound emerging adults is not well understood. Methods: This is a cross-sectional study of current and historical alcohol and marijuana use among college-bound recent high school graduates (N = 1,365; age ∼18 years). Results: Drinking was prevalent (78%, N = 1,055) and marijuana use was prevalent (46%, N = 622). Stepwise logistical regression revealed the lower the age of first use, the higher the prevalence of current problematic substance use. Those who initiate alcohol ≤ 12 are twice as likely to currently use marijuana frequently. A significant relationship was found between age of first use and non-social substance use (drinking, p = 0.0001; marijuana, p = 0.0025). The temporal ordering of substance use indicates that alcohol precedes marijuana use, and age of first alcohol use is relevant to rates of initiation and current marijuana use.