Fifty-eight adults with Down syndrome (ages 31 to 56 years at time of first testing, mean age, 43.5) were assessed longitudinally over 10 years for the purpose of modeling aging-related change in cognitive function and adaptive behavior. Cognitive function was assessed seven times using the Woodcock–Johnson Tests of Cognitive Ability-Revised Early Development Battery. Adaptive behavior was evaluated five times using the Inventory for Client and Agency Planning. Multi-level (hierarchical linear) modeling procedures were used to model change with age. Findings provided further evidence of changes in performance with age and included selected effects for participants who completed the 10 years of study and those who were lost to follow-up as well as for age cohorts.