One respected publication that addresses this complex question is the textbook Physical Activity Instruction of Older Adults (2005) in which SFA president Janie Clark wrote the chapter "Designing and Managing Group Conditioning Classes." In a chapter entitled "The Field of Gerokinesiology," co-editors C. Jessie Jones and Debra J. Rose explain the limited nature of relying solely on chronological years to describe old age (for example: young-old 65-74; middle-old 75-84; old-old 85-99; and oldest-old 100-plus). There is simply too much diversity within numerical age categories to form definitive profiles. Jones and Rose then discuss several other indicators of aging, including two we will briefly outline here: biological aging and functional age.
Also called primary aging, biological aging concerns a number of processes in the human body that, over time, result in reduced adaptability, disease, physical and functional declines, disability, and ultimately death. Numerous theories of biological aging — for example: genetic theories which emphasize heredity; damage theories which stress the long-term build-up of cell damage; and other theories — are presently under scientific investigation and debate.
Functional age refers to an individual’s functional fitness level, compared to others of his or her same chronological age and sex (for example: how much and what types of physical activity can one successfully perform? what is the status of one’s cardiovascular system? one’s musculoskeletal system? what are an individual’s capacities and/or limitations in terms of carrying out activities of daily living?). As a good example of gauging functional fitness, Jones and Rose cite influential researcher Waneen W. Spirduso’s well-known Hierarchy of Physical Function. Her publication Physical Dimensions of Aging (1995) separates physical function into five categories in descending order: physically elite; physically fit; physically independent; physically frail; and physically dependent. A second edition of Physical Dimensions of Aging was published in 2005.