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.
By that logic, there’s nothing guaranteeing iMessage on iPhones is secure or private either because it’s closed source. If you don’t want to trust Beeper mini, you’ll be free to run their iMessage bridge on your own Matrix stack when they open source it at some point, which they’re promising to do (and you still won’t know that Apple isn’t scraping your messages on the iOS side). When I decide to trust a company, it’s because I look at what they’re transparently communicating to their end users. Every indication is that they are trying to get out of the middle of handling encrypted messages. Their first move to make this happen was allowing people to self host their own Beeper bridges (which you can still do with Beeper Cloud if you prefer and you will know that your messages are always encrypted within the Beeper infrastructure). They aren’t going to release the source for their client ever because that’s the only way they make any money.