This study compared in-school and out-of-school behavioral health service use and associated expenditures among children of different ages and with different psychiatric disorders. Medicaid claims from one city provided a sample of 24,271 children aged 5-17 years with psychiatric disorders who received one or more Medicaid-funded behavioral health services between October 1, 2008, and September 30, 2009. Children were categorized as having autism, conduct disorder/oppositional defiant disorder, attention deficit hyperactivity disorder, and other psychiatric disorders. Logistic regression and generalized linear regression were used to estimate differences in in-school and out-of-school behavioral health service use and associated Medicaid expenditures by age group (5-11 years and 12-17 years old) within and across psychiatric disorders. Significant differences were found in Medicaid service use by age group and by disorder. Significantly fewer children 12 years of age or older used in-school and out-of-school behavioral health services than children 5-11 years of age. Among service users, Medicaid expenditures for out-of-school service use were higher for older children. In-school behavioral health service use decreased significantly with age and more than out-of-school services, suggesting the need for access to appropriate school services for adolescents. Differences in these changes by disorder may point to the need for disorder-specific strategies to provide appropriate care as children age.