If we tie the person's identity to the token (THIS IS A BAD IDEA) then if underage use is detected, the adult who sold the token can be prosecuted for selling adult tokens to minors.
No such friction or accountability can possibly exist when we're talking about a bunch of yubikeys, unless we attach identity to them (THIS IS A BAD IDEA).
The core point is that it isn't my responsiblity to take care of your children. When less than half of millennials (40% men, 55% women) have children, the "think of the children" catchphrase really starts to fall flat. Why should I think of the children? I don't even have children. It's the parents' problem, not mine. Stop making it my problem.