We are living longer than ever before in human history. But longer lives are at the same time a gift and a challenge for individuals and society alike. Longer lives highlight an extraordinary feature of the human species and, that is, the capacity to intentionally or unintentionally positively modify their own development and aging. This positive plasticity of human development and aging is based on the fact that human aging is neither biologically nor contextually determined. Instead, development and aging are the result of perpetual interactions between biological, sociocultural forces and a given person's behaviors. Rethinking adult development implies that research needs to intensify its efforts to investigate and uncover the conditions and mechanisms facilitating the positive plasticity of adult development and aging. We need to accumulate scientific knowledge about which trajectories of constellations of sociocultural and physical context characteristics, a person's behavioral patterns and genetic endowment are apt to optimize aging. Research examples from cognitive and personality functioning are presented to illustrate the positive plasticity of adult development as well as its limits. Cohort- and country-comparative long-term longitudinal data including physiological as well as behavioral measures besides sociodemographic information and information pertaining to the physical environment are needed to gain a deeper understanding of how to leverage the positive plasticity of human aging. Such evidence is then in a position to provide specific and therefore effective evidence to inform social policy as well as life(style) choices.