42.
I think a big part of this age-shape is that forums aren't really cool or exciting anymore.
If you're wanting to ask questions and get answers and so on, reddit tends to me more immediate (because of the massive numbers of users and it being essentially one giant super-forum), as do various Discords (albeit reddit is more likely to get you a correct answer than any non-specialist Discord). I mean, if I was new to the internet today, no way I'd go to forum, I'd probably be on reddit/Discord tons though.
20+ years ago, though, when the internet was still shiny and new? They were awesome, and they're still a format of communication that I like, because you can actually think about stuff (not that I cared about that 20+ years ago).