Old people often have limited regenerative abilities and are more susceptible to disease, syndromes, and sickness than younger adults.The organic process of ageing is called senescence, Definitions of old age include official definitions, sub-group definitions, and four dimensions as follows. At what age old age begins cannot be universally defined because it differs according to the context.
Other writers treat the perceptions of middle-age people regarding their own old age. Studies find that many people in the 55–75 range can postpone morbidity by practicing healthy lifestyles. Minois comments that the scribe's "cry shows that nothing has changed in the drama of decrepitude between the age of the Pharaoh and the atomic age" and "expresses all the anguish of old people in the past and the present." Lillian Rubin, active in her 80s as an author, sociologist, and psychotherapist, opens her book 60 on Up: The Truth about Aging in America with "getting old sucks. Rubin contrasts the "real old age" with the "rosy pictures" painted by middle-age writers. Morrison delineates the heroism required by old age: to live through the disintegration of one's own body or that of someone you love. been with us since the stage of primitive society; it was both the source of wisdom and of infirmity, experience and decrepitude, of prestige and suffering." "Beauty and strength" were esteemed and old age was viewed as defiling and ugly.
Therefore, rather than lumping together all people who have been defined as old, some gerontologists have recognized the diversity of old age by defining sub-groups.
One study distinguishes the young old (60 to 69), the middle old (70 to 79), and the very old (80 ).
In addition to chronological age, people can be considered old because of the other dimensions of old age.
For example, people may be considered old when they become grandparents or when they begin to do less or different work in retirement.