The only way I've seen writing/creative communities making this kinda work on Slack or Discord was to make private channels for adults and then a mod approves the user to access, but they cant make a user prove they are telling the truth about their age.
I joined a community like this once and I was way too uncomfortable being surrounded by minors because of the content of my story, so I left. Plus, I couldnt relate to any of their conversations because I do not have homework and I dont need permission from my parents lmao.
It will probably be the case with this one ya'll are starting, where it cant be helped who joins and if someone is uncomfy, they have the optiont to leave it.
Usually, writers from the tapas server who get to know each other over time will make a private little server with 10ish people who are adults.