Lemmy

What you're describing is only possible on de-anonymized platforms that essentially have "know your customer" type policies where users have to provide some kind of proof of their identity. While I agree that there is value in social spaces where everyone generally knows the people they're interacting with are who they say they are, I don't think this is ever going to be feasible in a federated social platform. I think Facebook is the closest thing we have to what you're describing, to be honest, and I believe Meta has even kicked around having a more sandboxed Instagram for minors (though I don't use Instagram, so I'm not certain on the details there).

For me, in most cases on a platform like Lemmy, a person's age is not something I care about. I care about what people are sharing and saying. But then again, none of my interests for online discussion at this point in my life are really age centric. I think there are clearly better platforms than Lemmy if people want to guarantee they're only interacting within their age specific peer groups.