This study examined differences by age in service use and associated expenditures during 2005 for Medicaid-enrolled children with autism spectrum disorders. Aging was associated with significantly higher use and costs for restrictive, institution-based care and lower use and costs for community-based therapeutic services. Total expenditures increased by 5% with each year of age; by 23% between 3-5 and 6-11 year olds, 23% between 6-11 and 12-16, and 14% between 12-16 and 17-20 year olds. Use of and expenditures for long-term care, psychiatric medications, case management, medication management, day treatment/partial hospitalization, and respite services increased with age; use of and expenditures for occupational/physical therapy, speech therapy, mental health services, diagnostic/assessment services, and family therapy declined.