Overview: Behavioural data reveal an inverted U-shaped function in the efficiency of prospective memory from childhood to young adulthood to later adulthood. However, prior research has not directly compared processes contributing to age-related variation in prospective memory across the lifespan, hence it is unclear whether the same factors explain the "rise and fall" of prospective remembering from childhood to later adulthood. The present study examined this question using a paradigm that allowed us to consider the behavioural and neural correlates of processes associated with the prospective and retrospective components of prospective memory. Methods: We compared 14 adolescents, 14 young adults, and 14 old adults in a paradigm where the prospective memory task was embedded in a semantic categorization task. Results: The behavioural data revealed an inverted U-shaped function with adolescents and old adults performing poorly relative to young adults. Analyses of the error data revealed that different processes may have contributed to failures of prospective memory in adolescents and older adults. This finding was supported by age differences in ERP-components for cue detection and post-retrieval processes. Additionally, source localization using LORETA revealed different patterns of neural recruitment for adolescents and older adults relative to younger adults. Conclusion: Our findings demonstrate that adolescents and older adults show different patterns of behavioural errors and neural recruitment for successful prospective remembering indicating that different processes may contribute to the "rise and fall" of prospective memory across the lifespan.