Introduction: Prospective memory is a robust predictor of functional capacity among older adults. Studies examining prospective memory and aging have suggested that prospective memory deficits are associated with aging. Although the impairment of prospective memory processes is mostly attributed to the impairment of the monitoring process, contradictory findings have been reported in the literature. This study aimed to determine the main factors underlying the negative effects of aging on prospective memory. To this end, we compared the monitoring performances of older and younger adults in time- and event-based prospective memory tasks using the eye-tracking method. Materials and Method: A total of 88 healthy and voluntary participants participated in the experiment. The time- and event-based prospective memory tasks were presented on a computer-screen. Participants were instructed to count the living/non-living objects, and when they saw the prospective memory target on the right corner of the screen, they were asked to press the spacebar on the keyboard. Results: A 2×2 analysis of variance was conducted. We found an age- related decline in event-and time-based prospective memory. In addition, the aging effect was greater in the time-based prospective Memory task, which requires more executive function than the event-based prospective memory task. The eye-tracking findings suggest that there is no monitoring deficit among older adults in either prospective memory task. Conclusion: We conclude that aging deficits in prospective memory tasks may not be due to monitoring deficits. Instead, executive functions other than monitoring are discussed as possible mechanisms underlying older adults’ reduced prospective memory performance.