What is Old Age?

Legal Definition
Old age refers to ages nearing or surpassing the life expectancy of human beings, and is thus the end of the human life cycle. In October 2016, a highly publicized paper claimed the maximum human lifespan is an average age of 115, with an absolute upper limit of 125 years, but the authors' methods and conclusions are controversial. Terms and euphemisms for old people include, old people (worldwide usage), seniors (American usage), senior citizens (British and American usage), older adults (in the social sciences), the elderly, and elders (in many cultures—including the cultures of aboriginal people).

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, the medical study of the aging process is called gerontology, and the study of diseases that afflict the elderly is called geriatrics. The elderly also face other social issues around retirement, loneliness, and ageism.

The chronological age denoted as "old age" varies culturally and historically. Thus, old age is a "social construct" rather than a definite "biological stage".

In 2011, the United Nations proposed a human rights convention that would specifically protect older persons.
-- Wikipedia
Legal Definition
This needs no definition. Sometimes old age is the cause of loss of memory and of the powers of the mind, when the party may be found non compos mentis. See Aged witness; Senility.
-- Bouviers Law Dictionary