Objective: Understanding developmental differences in reasons for quitting substance use may assist clinicians in tailoring treatments to different clinical populations. This study investigates whether alcohol-disordered and problem-drinking emerging adults (i.e., ages 18-25 years) have different reasons for quitting than younger adolescents (i.e., ages 13-17 years) Method: Using a large clinical sample of emerging adults and adolescents, we compared endorsement rates for 26 separate reasons for quitting between emerging adults and adolescents who were matched on clinical severity. Then age group was regressed on total, interpersonal, and personal reasons for quitting, and mediation tests were conducted with variables proposed to be developmentally salient to emerging adults. Results: Among both age groups, self-control reasons were the most highly endorsed. Emerging adults had significantly fewer interpersonal reasons for quitting (Cohen's d = 0.20), and this association was partially mediated by days of being in trouble with one's family. There were no differences in personal reasons or total number of reasons for quitting. Conclusions: Our findings are consistent with developmental theory suggesting that emerging adults experience less social control, which here leads to less interpersonal motivation to refrain from alcohol and drug use. As emerging adults in clinical samples may indicate few interpersonal reasons for quitting, one challenge to tailoring treatments for them will be identifying innovative ways of leveraging social suports and altering existing social networks.