Resistance to disease becomes progressively less with increasing age. Older individuals are killed by diseases which would not have destroyed them when they were young. It has been stated by Alex Comfort that, had we throughout life the same resistance to stress and disease as we had at the age of 10, about half of us might expect to live approximately 700 years. The object of studying the changes that occur with aging is to find out why they occur and what can be done about them.
Many of the changes which take place in the body and nervous system are susceptible of reasonably exact measurement. There is dispersion of energy with the passage of time. This results in structural and functional alterations which are the measure of rate of aging. When a tissue retains its ability to maintain optimum structure for function, the rate of physiological aging may be
Bortz EL. STRESS AND EXHAUSTION. JAMA. 1957;164(18):2059–2060. doi:10.1001/jama.1957.62980180008018