"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

"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