OBJECTIVE: Adults with intellectual and developmental disabilities (IDD) are living longer, yet research about the medical and psychiatric needs of older adults still lags behind that of younger individuals with IDD. The aim of this study was to assess age-related differences in the mental health presentations of adults with IDD. METHODS: Fully deidentified data for adults 30 years and older were extracted from the START (Systemic, Therapeutic, Assessment, Resources, and Treatment) Information Reporting System, a deidentified database housed at the Center for START Services. Caregivers and START team documents reported psychiatric diagnoses, service use, recent stressors, and challenging behaviors. t Tests, Mann Whitney U tests, χ2 tests, and multinominal logistic regression models were used to compare the two age groups, 30-49 years (n = 1,188) versus 50 years and older (n = 464). RESULTS: Older adults had more medical conditions, fewer reported psychiatric conditions, and were more likely to be taking more psychiatric medications compared to younger adults, after adjusting for demographic variables, disability level, and number of recent stressors. CONCLUSION: Although older individuals reported fewer psychiatric diagnoses, they were more likely to take higher numbers of psychiatric medications and have more medical conditions. Clinicians and researchers ought to devote more attention to the healthcare needs of older adults with IDD, a vulnerable group exposed to polypharmacy and at risk of adverse events.