Background Children with attention-deficit hyperactivity disorder (ADHD) have difficulties with executive function and impulse control which may improve with age. Aims To map the brain correlates of executive function in ADHD and determine age-related changes in reaction times and brain volumes. Method Attention-deficit hyperactivity disorder and control groups were compared on the change task measures of response inhibition (stop signal reaction time, SSRT) and shifting (change response reaction time, CRRT). voxel-wise magnetic resonance imaging (MRI) correlations of reaction times and grey matter volume were determined, along with bivariate correlations of reaction times, brain volumes and age.Results Individuals in the ADHD group had longer SSRTs and CRRTs. Anterior cingulate, striatal and medial temporal volumes highly correlated with SSRT. Striatal and cerebellar volumes strongly correlated with CRRT. Older children had faster reaction times and larger regional brain volumes. In controls, orbitofrontal, medial temporal and cerebellar volumes correlated with CRRT but not SSRT. Neither reaction times nor regional brain volumes were strongly age- dependent. Conclusions Our evidence supports delayed brain maturation in ADHD and implies that some features of ADHD improve with age.