Background: Most laboratory-based studies on prospective memory show a decline with increasing age. Theoretical explanations for age differences focus on the allocation of attentional resources to support prospective remembering. The recruitment of prospective memory target monitoring seems to be influenced by perceived task importance. Objective: In the present study, we investigated the influence of task importance on the magnitude of age differences in event-based prospective memory. Methods: Healthy younger (n = 25) and older (n = 25) adults were instructed a priori to prioritize either the ongoing or the prospective memory task before performing an event-based prospective memory task. Results: We found an interaction between age and task importance: instructed higher importance of the ongoing task compared to the prospective memory task component produced significant age-related declines in prospective remembering. By contrast, if older adults treated the prospective memory task component as more important than the ongoing task, they achieved equivalent levels of prospective memory performance as their younger counterparts, but did so at a cost to ongoing task performance. Conclusions: The present data indicate that task importance is one of the factors determining the presence or absence of age deficits in prospective remembering. Findings are discussed in the context of limited processing resources in old age and theoretical frameworks of event-based prospective memory.