I think this "reddit is full of teenagers" statement is a trope for two reaspons.

First, my partner is a highschool math teacher and in his casual surveys and discussions, most students don't use reddit: tiktok still dominates. The ones that do, mostly boys, just watch fight and war videos. Girls apparently are into makeup videos. News to me.

Second, I'm in my 50's and there's no way teenagers are interested in political discussions. Maybe they are, but the people I get in flame wars with speak and act like angry adults: their rages and complaints would require too much effort for a teenager to fabricate.

I think the truth is Reddit is mostly adults but for some reason lots of adults want to believe it is just teenagers, possibly because we act like children online sometimes?