The nature of time: implications for research on aging.

Contemporary research on aging gives little evidence of recognition of the underlying concepts of time. Researchers have been much concerned with age as a variable but little with the explication of aging as a process. Most authors do not define aging as they use the term or explain how time enters their causal explanations. In this article major concepts of time are reviewed and distinctions are made between physical, biological, psychological, and social time or age, which all might be classified as a variant of calendar time or calendar age, respectively. Recent developments in gerodynamics, however, have given rise to the concepts of intrinsic time and intrinsic age, which are generated by the dynamics of the system. The implications of the major concepts of time and age will be drawn for research on aging.