This is underselling it -- an adult can sell these yubikeys untraceably, pocket the cash, and never think about it again in their life.
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.
An adult could also sell alcohol or drugs to minors so what? Should we add a code to every bottle and require passport to buy them?
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).