Objective: This study explored whether age moderated cognitive, symptom, and functional changes over a 12-week compensatory cognitive training (CCT) intervention for participants with severe mental illnesses. CCT focused on the cognitive domains of attention, learning, prospective memory, and executive functioning, often impaired in this population. Method: Seventy-seven unemployed individuals (46 participants with severe mood disorders and 31 participants with schizophrenia/schizoaffective disorder; mean age = 44 years) received CCT for 12 weeks in the context of a supported employment program. Participants were administered cognitive, symptom severity, and functional measures at baseline and 3-, 6-, and 12-month follow-ups, as well as at 18 and 24 months for symptom/functional measures. Mixed effects models, controlling for diagnosis, examined whether age impacted the trajectories of change following CCT. Results: Analyses showed several significant time by age interactions; younger participants improved more over time on category fluency, beta =.-280, t(42.10) = -2.76, p =.008, and financial capacity (UCSD Performance-Based Skills Assessment), beta =-.194, t(54.02) = -2. 21, p =.031, whereas older participants showed greater reduction in positive symptom severity (Positive and Negative Syndrome Scale), beta =-.109, t(78.35) = -2.34, p =.022, and less functional decline on the Independent Living Skills Survey, beta =.118, t(109.77) = 2.05, p =.043. Conclusions and Implications for Practice: Age moderated the effects of CCT over time on measures of cognition, symptom severity, and functioning. Younger participants improved on objective measures of verbal processing speed and financial capacity, whereas older participants showed reduced positive symptom severity and less decline in self-reported daily functioning. These findings suggest that CCT may differentially benefit persons with severe mental illnesses depending on age.