In the last two decades, a burgeoning literature has begun to clarify the processes underlying personality traits and momentary trait-relevant behavior. However, such work has almost exclusively investigated these questions in young adults. During the same period, much has been learned about adult personality trait development but with scant attention to the momentary processes that contribute to development. The current work connects these two topics, testing developmental questions about adult age differences and thus examining how age matters to personality processes. The study examines how four important situation characteristics are experienced in everyday life and how situations covary with Big Five trait-relevant behavior (i.e., situation-behavior contingencies). Two samples were collected (total N=316), each assessing three age groups: young, middle-aged, and older adults. Using ESM, participants completed reports 4 or 5 times per day across a representative period of daily life. Results suggested age differences in how situations are experienced on average, in the variability around these average situation experiences, and in situation-behavior contingencies. The results therefore highlight that, across adulthood, age groups experience chronically different situations, differ in how much the situations they experience vary moment to moment, and differ in how much situation experience predicts their enactment of traits.