Adults can be prosecuted for selling alcohol to minors (I mean, they can be prosecuted for selling drugs to other adults even). Because there is a physical handoff, there is risk in doing so, and thus there is high friction, which we deem to be sufficient to curtail the risk.
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).
It is the easiest thing in the world to make it illegal. But while making the sale of alcohol to minors has some teeth, as above, the sale of these "tokens" is so low friction as to be laughable.
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.
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).