I'm just going to say this and hope that it makes enough sense to be understood.
You're on Wizardchan. You're on a website full of many, many exceptions to the rules as it is, as we all fly in the face of what's socially acceptable or appreciated. Age has never had anything to do with how we turned out, and I'm sure some of us undoubtedly knew as children that we'd end up like this (I know for a fact that I did).
Instead of trying to ban people based on their ages or focusing on that particular doomed idea, why not just ban people who make low quality threads and double down on that notion? It's far more productive, it's actually feasible, and you can actually tell when someone is being a low-effort nuisance who isn't interested in actually gelling with the community whatsoever and just uses this place as a garbage dumb away from their garbage dumps on 4chan and Reddit.
I understand the value of flying under the radar and keeping Wizardchan small but I don't think putting up even more regulations for posting is going to help, especially if they're as unrealistic as this. Surely in today's era, there are even more younger people like us solely because of the completely abhorrent social conditions? Why turn them away, as long as they're not using this place as their small-time /r9k/ or subreddit? Obviously, they should be 18+ but there's never been a way to verify that beyond someone outing themselves as being younger than that.
Besides, how many of us were actually that age when we started coming here? If I remember right, I was in my early 20s, and this place helped me through some very tough times. I'd hope we would be willing to keep extending such a helpful hand to a more and more disenfranchised, disenchanted youth.