You seem to have missed requirement #3 -> tracking and identifying reuse.
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.
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.