This article is based upon a chapter in a book to be called Britain! Be Your Age! It begins with a discussion of schemes for dividing the life-course, describes the fresh division to which the title the Third Age belongs and refers briefly to a general theory of the Third Age. It is claimed that the Third Age as thus defined did not emerge in Britain and other Western countries until the 1950s, nor did it become a settled feature of their social structures until the 1980s. Expectation of life in a number of countries, developed and developing, is contrasted, and a comparison is undertaken between life expectation in the contemporary Third World and that in England in the historical past, that is since the early 16th century. It is concluded that contemporary developing societies have much longer life expectation than that in the English past, but markedly fewer elderly people. The implications of this for the modernisation theory in relation to ageing are drawn out, and the concept of modernisation shown to be unacceptable to historical sociologists. A Third Age Indicator (3AI) is then suggested, expressing the probability of a person of 25 years attaining 70 years. The Third Age is defined demographically in a two-fold way, as a condition of a population in which the general expectation of living from 25 to 70 is 0.5 or over for men, and so more for women, and of 10% or more of the whole population being over age 65. 3AIs for a number of contemporary countries are then presented, along with those for England since the 1540s. A list of countries demographically qualified on the two counts is then drawn up, along with the appropriate dates of their attaining that status.