Your crypto nerd dream is vulnerable to the fact that someone under 18 can just ask someone over 18 to make an account for them. All age verification is broken in this way.
There is a similar problem for people using apps like Ubereats to work illegally by buying an account from someone else. However much verification you put in, you don't know who is pressing the buttons on the screen unless you make the process very invasive.
An 18-year-old creating an account for a 12-year-old is a legal issue, not a service provider issue. How does a gas station keep a 21-year-old from buying beer for a bunch of high school students? Generally they don't, because that's the cops' job. But if they have knowledge that the 21-yo is buying booze for children, they deny custom to the 21-yo. This is simple.
They don't? Teenagers can easily get their hands on alcohol... you just need to know the right person at school who has a cool older brother. If their older brother is really cool they can get weed too!
The police absolutely do not have the time to investigate the crime of making a discord account for someone.