The same can happen with any type of authentication, unless you are ok with someone pointing a webcam and authenticating you non stop.
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).
This is another of the many many solutions in the "the age limitations that exist currently are easily circumvented" class. While it may be effective in some limited ways, it's not clear that it's better than say parental controls. And it is obviously deficient compared to identity verification solutions that do not preserve anonymity. You can't pretend that this solution is "just as good" when it isn't. The identity verification solution is bad not because it is ineffective at its direct aims but because it has disastrous side effects.
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.