You could say either, preferably “among” of those two, but neither is a common preposition to sue with “famous”.

Much more common (by a ratio of 9:1) is “famous in”, though that requires not “teenagers” but something like “He is famous in US teenage pop culture.”

See https://lingohelp.me/preposition-after-adjective/famous-for-in-of-by-as/

@Zaku A bit rude but a necessary question. I need to know because it matters in the matter being discussed. Is English your first language?

Yes, it’s my first language.

I have never seen famous used with either term. You can say that someone is well known among teenagers, so among would be the better choice, since well known is closer to famous than popular is.