The decrements in physiological functions that take place with increasing age become manifest primarily when the environment places extra demands on the individual. The age-dependent reductions in reserve capacities result in part from a gradual loss of functioning tissue. Since complex and coordinated functions are more influenced by age than are simple cellular changes, it is concluded that age decrements in performance are also a result of a breakdown of neural and endocrine regulatory processes.
Shock NW. Current Concepts of the Aging Process. JAMA. 1961;175(8):654-656. doi:10.1001/jama.1961.03040080010003