The minimum number of trials necessary to accurately characterize the error-related negativity (ERN) and the error positivity (Pe) across the life span was investigated using samples of preadolescent children, college-age young adults, and older adults. Event-related potentials and task performance were subsequently measured during a modified flanker task. Response-locked averages were created using sequentially increasing errors of commission in blocks of two. Findings indicated that across all age cohorts ERN and Pe were not significantly different relative to the within-participants grand average after six trials. Further, results indicated that the ERN and Pe exhibited excellent internal reliability in preadolescent children and young adults after six trials, but older adults required eight trials to reach similar reliability. These data indicate that the ERN and Pe may be accurately quantified with as few as six to eight commission error trials across the life span.