Objective: Attention-deficit/hyperactivity disorder (ADHD) has been associated with widespread changes in cortical thickness (CT). Findings have been inconsistent, however, possibly due to age differences between samples. Cortical changes have also been suggested to be reduced or to disappear with stimulant treatment. We investigated differences in CT between adolescents/young adults with and without ADHD in the largest ADHD sample to date, the NeuroIMAGE sample. Second, we investigated how such differences were related to age and stimulant treatment. Method: Participants (participants with ADHD = 306; healthy controls = 184, 61% male, 8-28 years of age, mean age = 17 years) underwent structural magnetic resonance imaging. Participants and pharmacies provided detailed information regarding lifetime stimulant treatment, including cumulative intake and age of treatment initiation and cessation. Vertexwise statistics were performed in Freesurfer, modeling the main effect of diagnosis on CT and its interaction with age. Effects of stimulant treatment parameters on CT were modeled within the sample with ADHD. Results: After correction for multiple comparisons, participants with ADHD showed decreased medial temporal CT in both left (pCLUSTER = .008) and right (p CLUSTER = .038) hemispheres. These differences were present across different ages and were associated with symptoms of hyperactivity and prosocial behavior. There were no age-by-diagnosis interaction effects. None of the treatment parameters predicted CT within ADHD. Conclusion: Individuals with ADHD showed thinner bilateral medial temporal cortex throughout adolescence and young adulthood compared to healthy controls. We found no association between CT and stimulant treatment. The cross-sectional design of the current study warrants cautious interpretation of the findings.