The goal of the present study was to examine individual differences in the degree to which controlled attention is allocated towards a prospective memory (PM) task. Using a PM task that should require high levels of controlled attention in a sample of 138 young, middle-aged, and older adults, two subgroups of participants could be identified, i.e., participants who clearly demonstrated evidence for monitoring and those for whom no clear evidence for monitoring was revealed. A control group (n = 95) was tested to control for practice effects in the ongoing task. Differences between subgroups were examined in terms of age, PM accuracy, baseline ongoing task performance, and general negative mood. Nonmonitorers and monitorers differed in age (more older adults being nonmonitorers), ongoing task accuracy (a nonsignificant trend was observed here), PM task accuracy (both young and middle-aged/older monitorers were more accurate than nonmonitorers), and the number of reported depressive symptoms (nonmonitorers > monitorers). Moreover, results showed that even in nonmonitorers PM accuracy was above floor level, indicating that noticing and reacting to some of PM cues is possible without strongly investing in resource demanding monitoring processes.