Aging is presumed to disrupt self-initiated processing, and a time-based prospective memory task (i.e., action to be performed at a particular time) entails more self-initiated activities than an event-based prospective memory task (i.e., action to be performed to a critical event). Accordingly, older participants are predicted to be particularly bad in a time-based prospective memory task. However, the prediction is not always confirmed. Self-initiated activities entail central executive functioning. We therefore predicted the age deficit to emerge more clearly when the performance on the ongoing task also involved more central executive functioning. Time-based prospective memory among older adults collapsed when the complexity of the ongoing task increased. However, an age deficit was also obtained when the pacing of the event-based prospective memory task was high because of the general slowing of functioning by older adults.