The present study examines change in reports of daily, weekly, and monthly psychological distress over 20 years, and of negative and positive affect over 10 years, using data from the Midlife in the United States study. The study includes three waves of data collection on adults ranging from 22 to 95 years old. Cross-sectional findings reveal that older age is related to lower levels of psychological distress and negative affect and to higher levels of positive affect across each successive age group. Yet, longitudinal findings vary across younger, middle-aged, and older adults. Psychological distress decreases over time among younger adults (although only until age 33 for weekly reports), remains stable in midlife, and is stable (monthly) or slightly increases (daily and weekly) among older adults. For negative affect, levels decrease over time for younger and middle-aged adults, and only increase for the oldest adults for daily and monthly affect. Positive affect is stable over time among younger adults, but decreases in midlife starting in the mid-fifties. In conclusion, overall patterns of findings suggest that being old (assessed cross-sectionally) is related to higher levels of emotional well-being. Growing old (assessed longitudinally) is related to improvements in emotional well-being across younger and early middle adulthood, which mirrors cross-sectional findings. There is relative stability in later midlife, however, and continued stability or slight declines across older age. (PsycInfo Database Record (c) 2023 APA, all rights reserved).