Everyday prospective memory (PM), such as remembering medication intake or appointments, is crucial for independence and quality of life, especially in older age. However, little is known about how momentary stress and busyness as contextual factors differentially affect naturalistic PM. Forty younger (18–27 years) and 40 older adults (60–84 years) completed a 14‐day ambulatory assessment of various naturalistic PM tasks, alongside time‐contingent reports of stress and busyness. Multilevel structural equation modeling revealed within‐person effects: stress was related to lower time‐based but higher event‐based PM, while busyness was related to higher event‐based PM and self‐assigned intentions. Effects of stress but not busyness were stronger in older adults. Older adults performed comparably to or better than younger adults on most tasks. Importantly, accounting for busyness and stress eliminated age differences in time‐based PM. Findings highlight task‐specific and differential effects of stress and busyness and demonstrate how their daily fluctuations shape age‐related PM.