Chronological age may differ considerably from a person's functional age.
The distinguishing marks of old age normally occur in all five senses at different times and different rates for different persons.
However, for its study of old age in Africa, the World Health Organization (WHO) set 50 as the beginning of old age.
At the same time, the WHO recognized that the developing world often defines old age, not by years, but by new roles, loss of previous roles, or inability to make active contributions to society.
The distinguishing characteristics of old age are both physical and mental.
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 ).
Some dictionaries describe widespread use of "senior citizen" for people over the age of 65.
When defined in an official context, senior citizen is often used for legal or policy-related reasons in determining who is eligible for certain benefits available to the age group.
Old age comprises "the later part of life; the period of life after youth and middle age . Most developed-world countries have accepted the chronological age of 65 years as a definition of 'elderly' or older person.