As the field follows recommendations to introduce standardized assessments on substance, mental, and behavioral problems, a consistent picture has emerged that co-occurring disorders are common, that there is heterogeneity in the type of disorder, and that the pattern varies by age. This study examines the prevalence of self-reported substance use and mental health problems, the pattern of comorbidity, and how both vary by age among people presenting to substance abuse treatment. Data are from 4,930 adolescents and 1,956 adults admitted to substance abuse treatment in multisite studies who were assessed with the Global Appraisal of Individual Needs and categorized into five age groups: <15, 15-17, 18-25, 26-39, and 40+ years. Two thirds of clients had a co-occurring mental health problem in the year prior to treatment admission. Across all ages, clients self-reporting criteria for past-year substance dependence were more likely than those who did not to have other co-occurring mental health problems (odds ratios of 2.9 to 8.8). The prevalence and patterns of co-occurring mental health problems, however, varied by age. Young adults (ages 18-25) were found to be most vulnerable to co-occurring problems.