Youth substance use disorder treatment programs frequently advocate integration into 12-Step fellowships to help prevent relapse. However, the effects of the predominantly adult composition of 12-step groups on adolescent involvement and substance use outcome remain unstudied. Greater knowledge could enhance the specificity of treatment recommendations for youth. To this end, adolescents (N = 74; M age = 15.9, 62% female) were recruited during inpatient treatment and followed up 3 and 6 months later. Greater age similarity was found to positively influence attendance rates and the perceived importance of attendance, and was marginally related to increased step-work and less substance use. These preliminary findings suggest locating and directing youth to meetings where other youth are present may improve 12-step attendance, involvement, and substance use outcomes.