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 paper 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 4,939 adolescents and 1,958 adults presenting to substance abuse treatment in multi-site studies who were assessed with the Global Appraisal of Individual Needs (GAIN) and categorized into five age groups: age <15, 15-17, 18-25, 26-39, and 40+. Two thirds of clients had a co-occurring mental 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 cooccurring mental health problems (odd ratios of 2.9 to 8.8). The prevalence and patterns of cooccurring mental health problems however varied by age. Young adults (age 18-25) were found to be most vulnerable to co-occurring problems.