"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](http://www.poetryfoundation.org/poem/173536)