As Eeyore said, I'm not even sure it's really aging out or the old board format. When no new person has been registered in years it's hard to keep going. There's even people who just can't post here anymore because they forgot their password so can't post.
Discord is pretty rough since groups there are having to bounce from server to server since Discord is getting into the private servers and banning them any time they think there might be a pedophile discord server. Makes it hard to build a membership for any of them. I know there are a few groups on Mastadon though with memberships in the hundreds. The Discords can manage to get that if they stay alive for a few weeks. Same with some telegram groups. Reddit also when the users can stick to just talking about a loli anime. Go off topic and mention you're a pedophile gets you a ban though.