Advancing age is often characterized by preserved or even enhanced emotion regulation, which is thought to manifest in terms of age-related reductions in the within-person association between stressors and negative affect. Existing research from ecological momentary assessment and end-of-day daily diary studies examining such age-related benefits have yielded mixed results, potentially due to differences in samples, design, and measurement of everyday stressors and negative affect. We conducted a coordinated analysis of 5 ecological momentary assessments and 2 end-of-day daily diary studies to examine adult age differences in the within-person association between everyday stressors and negative affect. Reported stressor occurrences are robustly associated with higher negative affect, regardless of study design and sample characteristics. Across studies, interactions between age and everyday stressors predicting negative affect revealed a pattern of age-related decreases in the stressor-negative affect association, but this interaction was only significant for 2 studies. Further, examination of statistical power of the included studies suggests that, despite differences in the number of repeated assessments, power to detect within-person stressor-negative affect associations is quite good. In contrast, despite possessing wider age ranges, observed age differences were relatively small in magnitude, and studies are potentially underpowered to detect age differences in these within-person associations. We discuss the importance of study design, interval of repeated assessments and number of participants for examining age differences in everyday stressors and negative affect, as well as the virtue of coordinated analyses for detecting consistent direction of associations, but inconsistent patterns of statistical significance. (PsycINFO Database Record (c) 2019 APA, all rights reserved).