Explaining Variation in Individual Aging, Its Sources, and Consequences: A Comprehensive Conceptual Model of Human Aging.

We define aging as a characteristic deterioration in one (or more) observable attributes of an organism that typically occurs during later life. With this narrow functional definition, we gain the freedom to separate aging from other processes of age-related change (e.g., maturation, growth, illness, terminal decline). We introduce a structural model that distinguishes between (1) the phenomenon of aging, (2) the subjective experience of aging, (3) sources of aging, and (4) consequences of aging. A core focus of the model is on the role of buffering mechanisms of biological repair and personal adaptation that regulate the relations between sources of aging, aging proper, and its consequences. The quality and level of functioning of these buffering mechanisms also varies across the life span, which directly affects the sources of aging, resulting in either resilience against or accelerated aging, and thus can be considered to be a major source of the variation in aging processes among different individuals. External factors comprising attributes of the physical environment and sociocultural characteristics are considered as contexts in which aging occurs. These contextual factors are assumed to feed into the various components of the model. Our model provides an interdisciplinary account of human aging, its sources and consequences, and also its subjective experience, by integrating biological, psychological, lifestyle, and sociocultural factors, and by specifying their interrelations and interactions. The model provides a comprehensive understanding of individual human aging, its underlying processes, and modulating factors. It allows for the derivation of empirically testable hypotheses, and it helps practitioners to identify elements that lend themselves to targeted intervention efforts aimed at increasing the resilience of individuals against aging and buffering its negative consequences.