This study examined the developmental trajectories associated with response inhibition and error processing as exemplar executive processes. We present fMRI data showing developmental changes to the functional networks underlying response inhibition and error-monitoring, comparing activation between adults and young adolescents performing the sustained attention to response task (SART). During successful inhibitions, we observed greater activation for the young adolescents than for the adults, in a widely distributed network including frontal, parietal and medial regions. When inhibition failed, however, adults showed increased activation compared to young adolescents in a number of regions, including bilateral parahippocampal gyrus, left and right lingual gyri, the right insula, and cerebellar regions. These differences largely remained even when the two groups were matched for performance, suggesting that performance differences are unlikely to be the driving factor behind these developmental differences. Instead, the neurodevelopmental trajectory of these important executive functions may reveal the basis for the immature executive functioning of the young adolescent.