Simple: you get banned from the specific instance where you’re violating the rule, like OP did.

It sucks, but this is literally how it’s supposed to work. They can participate everywhere except where they are violating the rules.

Realistically, all instances should probably be restricting access to 18+ so they don’t need to navigate the mess of US’s COPPA (I think that’s the name) laws regarding retention of minors’ data. I’m sure other jurisdictions have laws around that as wellm

And a free tip from someone a decade and a half past the 18 line: Don’t let people know how old you are online if you’re underaged. It’s just asking for trouble. We all lied about it when we were in your shoes. The ones who didn’t had to deal with creeps.