Little research has examined social development in the young adult years relative to childhood and adolescence. This study tested the hypothesized pathways of the social development model (SDM) in young adulthood for predicting symptoms of alcohol use disorder (AUD) and positive functioning at age 30. A longitudinal panel study originally drawn from Seattle, Washington, elementary schools was examined. The sample included 808 participants with high retention and was gender balanced and ethnically diverse. Analyses focused on ages 21, 27, and 30. SDM constructs were assessed with self-reports of past-year behavior and combined multiple life domains. AUD symptoms corresponding to the Diagnostic and Statistical Manual of Mental Disorders, fourth edition (American Psychiatric Association, 1994) criteria were assessed using the Diagnostic Interview Schedule. Positive functioning combined measures of constructive engagement in work and school, civic engagement, physical exercise, and lack of depressive symptoms. The study found that AUD symptoms were moderately stable from age 21 to 30; however, developmental pathways hypothesized by the SDM at age 27 played a significant role in partially mediating this association. Alcohol-specific factors were key mediating mechanisms, whereas prosocial factors played little role. Conversely, prosocial factors had an important role in predicting positive functioning at age 30, whereas there were no significant pathways involving alcohol-specific factors. Findings suggest that age 27 is not too late for interventions targeting adult social development to help diminish alcohol use disorder symptoms by age 30. Alcohol-specific factors such as reducing perceived opportunities or rewards for heavy alcohol use or challenging beliefs accepting of drunkenness are likely to be key ingredients of effective adult interventions.