Older adults often perform worse than younger adults on prospective memory tasks, but the exact mechanisms remain elusive. Participants had to indicate whether a letter in the center of gaze is a vocal or a consonant (ongoing task). The prospective memory task was to first push a button if the letter was a certain character (relevant for ongoing task) or had a certain color (irrelevant for ongoing task). Targets were presented either in the center of gaze, or in the right periphery, requiring an eye movement to be detected. Older adults performed worse than young controls when the target attribute was irrelevant for the ongoing task or was presented in the periphery. Eyetracking showed that older adults generally monitor well, but often fail to initiate the appropriate action although they look at the target. Our findings are strong evidence against pure monitoring deficits and point to post-retrieval processes as the main source of impaired age-related prospective memory performance.