"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)