Since is used to refer to a point of time in the past and for is used with time interval. However I've come across this phrase
I haven't seen you since ages.
Is this a correct usage of since since ages denotes an interval of time?
There are two different patterns mixing here: "I haven't seen you since last year" and "I haven't seen you for ages"
As evidenced by work in Conversation Analysis, people actually break their own line of thought quite a lot, so it's not uncommon for people to switch wordings mid-stream.
A novel writer might express it like so: "I haven't seen you since ... ages", to highlight the supposed realization by the speaker that "I don't know when I last saw her, so I can't finish this sentence." But in real speech, people do this kind of switching without skipping a beat.
Since ages is most unusual. There are no records for it in the British National Corpus and only two in the Corpus of Contemporary American English.
Where did you see or hear it? Was it produced by a native speaker?
I think the correct phrase is for here. Although since could be used colloquially but it'd not be the correct way. Since is usually used when it refers to an exact or precise point in time. Ages is not an exact point in time. Here is an example.
I haven't seen you since last year.
I'm familiar with the "since ages" usage. To comment on Andreas's answer, I think the ellipsis is a nice theory, but in colloquial conversational contexts people really do sometimes say "I haven't seen you since ages" perhaps to emphasise the emotional content of the remark by being purposefully agrammmatical.
The usage is also familiar in variants of the phrase since ages long gone.
Besides, why do romantic minds become so enthralled about the case of the tepuis? After all, no matter how spectacular they are, they are no more 'islands in time' than ordinary islands in the seas which have been separated from their continents since ages long gone by.
From The Natural History of Hidden Animals, emphasis added.
Another similarly grand phrase is "since its days of origin"; and another very colloquial one is "since forever".