"in ages" or "for ages"?

For is correct ... because it is discussing time ...
I have not seen you for a long time.
"I haven't seen you for ages" means "I haven't seen you for a very long time".
"For ages" is a set expression meaning a long period of time.
Someone uses "I have not seen you in ages". But it is not the habbit ... and
using in to describe how long, is incorrect.
SO, FINAL ANSWER : HAVEN'T SEEN YOU FOR AGES !

以上是文法的分析，
口語上，
haven"t seen you for ages...

no problem.

for ages

正確用法為in ages。

I have not seen you in days.
I have not seen you in months.
I have not seen you in years.
I have not seen you in ages.

但若是量詞前有個數詞，preposition就要變成for：

I have not seen you for three days.
I have not seen you for two months.
I have not seen you for ten years.