INTRODUCTION: Forming an intention is a key aspect of prospective memory, i.e., the ability to encode, retain, and later realize an intention with a delay of minutes, hours or days. Behavioural and neurophysiological findings from both prospective and retrospective memory research suggest that the efficiency of encoding processes is reduced at both ends of the lifespan and that neural generators underlying successful encoding might differ in childhood and old age. Hence, the present study investigates compensational neural mechanisms during the encoding of intentions in adolescents and old adults compared to young adults. METHODS: We compared Event-Related Potentials (ERPs) and their source localization in 14 adolescents (11-13 years), 14 young adults (18-25 years), and 14 old adults (64-79 years) in a prospective memory task that was embedded in a semantic categorization task. RESULTS: Our data revealed three event-related modulations that differentiate between conditions (i.e., ongoing activity and successful intention formation trials) and groups. Source localizations of these modulations with standardized low-resolution electromagnetic tomography (sLORETA) revealed compensational activations in adolescents and old adults compared to young adults in successful intention formation trials: while adolescents showed a higher activation of secondary occipital regions in the time window of 500-1200 msec with a maximum around 800 msec, old adults activated prefrontal regions to a greater extent beginning at 700 msec, persisting until 1200 msec and expanding to middle temporal regions. CONCLUSION: For a successful encoding of intentions adolescents and old adults recruit more neural generators than young adults. More importantly, the pattern of these compensational activations is different when comparing adolescents with young adults and old adults with young adults. These differences are discussed with regard to differential maturational changes in the brain.