This article aims to (a) describe the study design of a 6-year follow-up multidisciplinary research project on aging, (b) report the psychosocial characteristics of the sample in detail, and (c) evaluate aging-related changes of health, physical activity, and psychosocial characteristics in 10 young-old (age at pre-test: M +/- SD = 63.2 +/- 1.5) and 12 old-old (age at pre-test: M +/- SD = 69 +/- 2) individuals. Both age groups consist of individuals displaying a high health status, a high extent of physical activity, high levels of psychosocial properties in the dimensions of well-being, life satisfaction, self-concept, body image, self-esteem, and self-efficacy, as well as a low general depression index. Psychosocial characteristics demonstrated a stable pattern over a period of nearly 6 years in both age groups with the exceptions of physical activity, satisfaction with children, general depression, and self-efficacy. Furthermore, physical self-concept decreased in old-old adults, whereas the young-olds showed no change. We assume that a high psychosocial status and a physically active lifestyle play an important role for mastering aging successfully in two life phases, each of which has its own challenges for older individuals. The decline in the physical self-concept of old-olds is interpreted as a first sign of subjective aging. Its association with losses in physical performance should be addressed in future studies. Finally, aging-related changes should be monitored on an individual level in order to capture the complex dynamic of aging that is not considered in analyses of between-person differences or averages.