In this study we examined the development of three action monitoring event-related potentials (ERPs) – the error-related negativity (ERN/Ne), error positivity (PE) and the N2 – and estimated their neural sources. These ERPs were recorded during a flanker task in the following groups: early adolescents (mean age = 12 years), late adolescents (mean age = 16 years), and adults (mean age = 29 years). The amplitudes of the ERN/Ne and N2 were greater in the adult and late adolescent groups than in the early adolescent group. Both of these components had neural sources in the anterior cingulate cortex (ACC). Although PE was present across groups, PE amplitude was greater in the late adolescent group compared to the adult group and also had neural sources in the ACC. ERN/Ne amplitude was related to post-error slowing across age groups; it was related to task performance only in the adult group. These findings are discussed in light of the role of the maturation of the ACC in the development of action monitoring processes.