Origin of “Third Age”

A recent question on ELU made me realize that the idea of “Third Age” as defined below is used mainly in BrE and other European languages, while AmE speakers appear to be unfamiliar with it.

the period of time that is after middle age and before old age, when people are still active. (MacMillan Dictionary)

Form the extract below it appears that the idea started in France but was actually carried out on a larger scale in England:

University of the Third Age

​an international organization aimed at providing 'lifelong learning for older people'. It organizes courses and other activities for retired people. It started as part of a university in France in 1973 and the first U3A group started in the UK in 1983 after the creation of a charity The Third Age Trust. Most courses in the UK are taught by U3A members themselves using their knowledge and experience. (OLD)

Where did the expression originate? Is it originally BrE or does it come from other European languages? What’s the AmE equivalent expression for “third age”?

Solution 1:

OED puts it firmly in the "translation from French" camp:

third age n. [translating French troisième âge] the period in life of active retirement; old age.

1972 Times 16 Mar. 13/7 We have devised a package deal for elderly people from the Continent... We are attempting to attract some Belgian old age pensioners. In Belgium it is called the third age.

It certainly isn't from Shakepeare's "Seven Ages of Man" ("All the World's a Stage" from As You Like it), although that might perhaps be expected. There, it's akin to his sixth age:

... The sixth age shifts
Into the lean and slippered pantaloon,
With spectacles on nose and pouch on side;
His youthful hose, well saved, a world too wide
For his shrunk shank, and his big manly voice,
Turning again toward childish treble, pipes
And whistles in his sound.

Solution 2:

The French Wikipedia has the following entry (my own translation, with apologies).

The third age is an abstract and temporally variable notion synonymous with old age, which goes back to the classical ages of life. People attaining the third age are generally called seniors or ‘elderly people’ . With the increase in life expectancy, as well as in the number of centenarians and ‘supercentenarians’, we have seen the increased use since the 1980s of the expression ‘fourth age’ ....

According to the Cambridge English dictionary, supercentenarian is a person of 110 plus years.

The ‘classical’ three ages must, I think, refer to the tripartite Roman words iuvenis (a ‘youth’), vir (a man - of military age), and senex (one too old for the army, an ‘oldster’). Life expectancy then was much lower: discounting infant mortality, it was about 50 years.