To characterize the development of neural substrate for interference processing and task control, this study examined both linear and non-linear effects of age on activation and connectivity during an interference task designed to engage the posterior medial frontal cortex (pMFC). Seventy-two youth, ages 8–19 years, performed the Multi-Source Interference Task (MSIT) during functional magnetic resonance imaging (fMRI). With increasing age, overall performance across high-interference incongruent and low-interference congruent trials became faster and more accurate. Effects of age on activation to interference- (incongruent versus congruent conditions), error- (errors versus correct trials during the incongruent condition) and overall task-processing (incongruent plus congruent conditions, relative to implicit baseline) were tested in whole-brain voxel-wise analyses. Age differentially impacted activation to overall task processing in discrete sub-regions of the pMFC: activation in the pre-supplementary motor area (pre-SMA) decreased with age, whereas activation in the dorsal anterior cingulate cortex (dACC) followed a non-linear (i.e., U-shaped) pattern in relation to age. In addition, connectivity of pre-SMA with anterior insula/frontal operculum (AI/FO) increased with age. These findings suggest differential development of pre-SMA and dACC sub-regions within the pMFC. Moreover, as children age, decreases in pre-SMA activation may couple with increases in pre-SMA–AI/FO connectivity to support gains in processing speed in response to demands for task control.