Monitoring the environment for the occurrence of prospective memory (PM) targets is a resource-demanding process that produces cost (e.g., slowing) to ongoing activities. Prior research has shown that older adults are able to monitor strategically, which involves the activation of monitoring when contextually appropriate and deactivation of monitoring when it is not thereby affording conservation of limited-capacity attentional resources. However, the time course and efficiency with which these processes operate with increased age are unknown. In the current study, participants performed an ongoing lexical decision task in which words/nonwords were blocked by font color in sets of ten trials (ten red trials followed by ten blue trials). Importantly, participants were informed that PM targets ("TOR" syllable) would only occur in red trials. Replicating previous work, both younger and older adults were successfully able to disengage monitoring upon encountering the unexpected (i.e., blue) context. However, while younger adults completely disengaged monitoring in the unexpected context, older adults continued to show monitoring across the majority of trials. Additionally, younger, but not older, adults showed a re-engagement of monitoring at the end of the unexpected context in preparation for the upcoming expected context. These findings suggest that while strategic monitoring generally remains intact with increased age, the disengagement and preparatory re-engagement of strategic monitoring may operate less optimally for older adults.