phrases - What is the origin of the term "ages"

That particular usage is hyperbolic, of course.

Age refers to long indefinite period of time, and in particular, it often refers to a lifetime or generation. This can be exemplified by collocations such as Age of Innocence or Quidditch Through the Ages.

In that respect, it is analogous to the Latin saeculum:

A saeculum is a length of time roughly equal to the potential lifetime of a person or the equivalent of the complete renewal of a human population. The term was first used by the Etruscans. Originally it meant the period of time from the moment that something happened (for example the founding of a city) until the point in time that all people who had lived at the first moment had died. At that point a new saeculum would start. According to legend, the gods had allotted a certain number of saecula to every people or civilization; the Etruscans themselves, for example, had been given ten saecula.

[Wikipedia]

Etymonline will tell you that the word had been used in this sense long before it was adopted in English, so I would consider an earliest known use immaterial here (and, it appears to be related to eon).