Concepts of Ages before the 20th Century...

c 700 BC: The Five Ages of Man

In his Works and Days [c 700 BC] the Greek poet Hesiod gives us the story of the Five Ages of Man. These are in no way astrological but refer to races of men: gold, silver, bronze, noble, and finally our own, "race of iron" who, "never rest from labour and sorrow by day, and from perishing by night; and the gods shall lay sore trouble upon them."

c 360 BC: Perfect Number

Writing in The Republic, [paragraph 546, Timarchy] [c 360 BC], Plato [c 427 - 347 BC] mentions that for "the divine creature, there is a period defined by a perfect number." Some commentators have considered this to be a reference to the Great Year, but in fact it's much more likely that the perfect number is 28, a reference to the number of days taken for the Moon to go through its changes of phase, the Lunar Month

c 360 BC: Complete Year

Writing in Timaeus, [paragraph 39c-d] [c 360 BC], Plato [c 427 - 347 BC] mentions the concept of a "Complete Year." This is the time period when "when all the eight circuits, with their relative speeds, finish together and come to a head, when measured by the revolution of the Same and Similarly-moving." This obscure-sounding sentence is a reference to the period of time it takes for all the planets to return to their original positions, but unfortunately Plato never tells us what he thinks these original positions were. This is not the same as a 'Platonic' Year, or Great Year, as the 'Complete Year' would be hundreds of thousands of years in length, even considering just the planets that would have been known to Plato [i.e. not including Uranus, Neptune and Pluto].

c 1778 AD: Great Year

In c 1778 Voltaire [1694 - 1778 AD] publishes Lettres Philosophiques, in which he looks back to the earlier work of Isaac Newton. He notes that, "hence it is that the ancients, who were doubly deceived, made their great year of the world, that is, the revolution of the whole heavens, to consist of thirty-six thousand years," which seems to be the first reference to the concept of a Great Year. Voltaire also describes a 'Platonic' Month concept, though he does not give it that name.