Biological and Mental ChangesLATEB PERIODS of life are chiefly marked by a decline in most constituents and traits of the person. The onset and fate of deterioration varies, however, from one organic system to another, from one person to another, and from one culture to another. The sequence depends upon many factors, especially hereditary endowment and specffic past experiences, such as illnesses and injuries. The process of aging is closely related to personal and social adjustment during the years of adulthood. Satisfactory adjustment in adulthood promotes the integration and maturity which aid in maintaining health and the characteristics of middle age.The time of onset of old age is difficult to define. Some persons reveal decided tiaits as early as forty; others appear still "young" at seventy. Just as no single criterion can be employed to delineate adolescence and adulthood, no decisive criterion can be given for the onset of old age. Individual variation is great indeed within each community and nation. Generally the mentally deficient, those with borderline deficiency, and physicaUy handicapped individuals deteriorate early, and often at a fast rate. At the age of thirty or even earlier, defectives begin to exhibit signs of old age. "Their life expectancy is very low. On the other hand, many well-endowed persons seem to be capable of resisting the oncoming process of decline up to sixty and seventy. Although the spry old person is in a much better physical condition than most persons who are several years younger, he is not so strong of vigorous as he was earlier, though he still manages to maintain a youthful attitude. The average person takes the middle ground. He begins to deteriorate earlier than the well-endowed person, yet his rate of decline is moderate. Ulti-291