>>779873In practice this isn't really true. Sure, "everyone" is on Twitter, but Twitter allows you to follow and block whoever you want, so in practice it turns into distinct communities. Reddit is even more segregated because everything happens within a single subreddit that's generally dedicated to a single subject with a specific vibe in mind, and in practice that selects for age too.
Typically in old forums you'd have a board dedicated to whatever the thing attached to the forum was – a game, a webcomic, a special interest, whatever – but then you'd have within the same forum a music board, a shoot-the-shit board, etc. and you'd get to mingle with the same people without the requirement of strict topicality. So it didn't really feel separated. And most old forums that I used had people as young as 13 (that was the legal age requirement) mixing with people as old as 60. Discord servers are the closest thing but in practice they're still more segregated because most of them aren't public.