At that point, what if parents just let their kids borrow their driver's licenses to use social media? There's no technical solution to bad parenting.

The only reasonable solution that doesn't infringe on privacy is to give parents the tools to limit their children's internet use, and presume, outside those bounds, that people are adults.

of course there's no perfectly privacy preserving solution for this, but ... zero-knowledge proofs have come a pretty long way.

if I understand correctly it's possible to give 16+ people tokens and then they can make the signups (transactions with these tokens) and then check that the transaction is valid (that it came from some valid token without knowing which token), while also making sure that folks can't just fake spend someone's tokens -- this is how the new Monero version is going to work after all.

https://www.getmonero.org/2024/04/27/fcmps.html

Of course as others mentioned trading identities (tokens) is trivial. (As I expect not-yet-16 olds will start stealing identities/logins of older people.)