There’d have to be a way of checking people’s identity, and thus their age, whenever they create an account, and it would have to be legally mandated for any online sites where people can post user-generated content.

Older siblings, friends, and organized students would just create multiple accounts and hand them over to younger kids. Also, a lot of adults would just create accounts for their kids.

It would be a keep off the grass sign no matter who implements it.