The human brain changes fundamentally over the course of life, however, whether such changes also impact on mechanisms of learning has remained uncertain. Here, visual perceptual learning, which refers to an enhancement of a visual skill through repeated experience or training, was used as a model to investigate whether mechanisms involved in the stabilization of learning during and after training differ between children (8 – 11 years old) and adults (18 – 35 years old). Participants (13 children, 14 adults) trained on a visual task that required them to detect the presence or absence of a visual orientation embedded in visual noise. Before, during and after the end of training, inhibitory (GABAergic) processing, which is associated with the stabilization of learning, was measured in early visual brain areas using functional magnetic resonance spectroscopy. The results showed that visual training triggered an increase of GABA only in children that persisted for several minutes after training ended. This finding predicted that training on new items rapidly increases the concentration of GABA in children and allows the learning to be rapidly stabilized. Subsequent behavioral experiments using new participants showed that children (n = 28) indeed stabilized new visual learning much more rapidly than adults (n = 28), which agrees with the common belief that children outperform adults in their learning abilities. Taken together, the results suggest that inhibitory, GABAergic processing during and after training is a key mechanism involved in effective and efficient learning in children.