There's no difference in meaning, both are fine. I hear "for ages" much more frequently by British English speakers, and "in ages" more commonly spoken by American English speakers.

Honestly, you could use either one. There is no difference in meaning between them. They are both used with about the same frequency.

both work but are a bit colloquial, informal, casual. I would say "I have not seen you for a long time" or "i have not seen you for such a long time" ('such' increases the timescale )

IN AGES is used after negative and superlative structures; and never used with future tenses. e.g. I haven't seen him in ages. or I haven't seem him for ages. (same meaning) e.g. This is the best movie I've seen in ages. (here you can't replace IN AGES for FOR AGES) I'm not a native speaker however most ESL grammar books and courses mention FOR AGES in their examples and drills so I guess FOR AGES is more frequent.

Thank you!