Combining behavioral and electrophysiological measures, we investigated the role of memory processes for prospective memory development in three different age groups over the lifespan. We focused on age differences during intention encoding, retention and retrieval in order to assess if potential age-associated performance differences in adolescence and older age can be explained by associated neurophysiological differences. Our research aim was to understand the impact of memory-related factors such as intention load and encoding time on prospective remembering, focusing especially on encoding and retention, which are two so far scarcely investigated phases. Adolescents, younger and older adults worked on a semantic judgment task with an embedded prospective memory task. Participants had to encode either one or two intentions; the encoding time was either four or eight seconds long. Younger and older adults outperformed adolescents behaviorally. Furthermore, performance was better for remembering one intention compared to remembering two intentions. On the neural level, we found age-specific modulations for the fronto-polar slow wave (FPSW) and the temporal-parietal slow wave (TPSW) that were sensitive to the number of intentions. Adolescents showed differences between encoding one or two intentions in the FPSW, while older adults showed these differences for the TPSW. Maintaining an intention increased fronto-central sustained activity compared to no intention. Furthermore, the activity during intention maintenance was sensitive to the number of intentions. Prospective positivity amplitudes during retrieval were smallest in adolescents and largest in older adults, but were not influenced by the memory manipulations. Parietal slow wave activity increased with increasing number of intentions, reflecting post-retrieval coordination between the ongoing and prospective memory task. In sum, only activity of the FPSW and the TPSW showed that age-related differences were influenced by memory-related factors during encoding, whereas these interactions were not observed for retention or retrieval. Our findings suggest that intention encoding and its efficiency play an important role in explaining age differences in prospective memory.