I understand obviously that an "age" is a measurement of time, but can someone specify for me the earliest known use of "ages" as a slang term? An example would be the following use:
The drive to the school took ages
Why is "age" the unit of time that is used in this case? Why not "eons" or "periods" or "cycles" or "centuries"? Thanks in advance.
"ages", as you describe, doesn't seem to be a slang word. In the context you present it means "a long time". Google Ngram viewer shows ages being used since the early 1800s.
e.g. "I haven't seen them in ages." = in a long time
Frost used it in "The Road Not Taken".
I shall be telling this with a sigh
Somewhere ages and ages hence:
Two roads diverged in a wood, and I—
I took the one less traveled by,
And that has made all the difference. -- Robert Frost
Consider age as a geological time period:
"A geologic age is a subdivision of geologic time that divides an epoch into smaller parts. A succession of rock strata laid down in a single age on the geologic timescale is a stage."
A geological time period is a long time, and thus so is an age.
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.
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).
