Biogerontologic theories.

Theories of age and aging engage a wide variety of phenomena and levels of explanation. There is a general consensus that aging is a complex process or set of processes, involving many "causal" inputs and manifold consequences. Two general theoretical orientations that explicitly address complexity are sketched here-the differential theory of quantitative genetics and systems theory. Among the suggestions derived are that it may be advantageous to consider aging to be hierarchically organized, with the corollary that subsystems of an organism can have different functional or biological ages, and that several or many indices will be required to provide an adequate characterization of a single individual. Aging probably proceeds by saltation rather than continuously. Uncertainty associated with bifurcations in complex systems, together with individual differences in timing and magnitude of step changes, may constitute fundamental limitations to the predictability across the full life trajectory. Genetic and environmental influences will differ from hierarchical subsystem to subsystem, and may differ within a subsystem across chronological age.