[pcrh]

A system as above would matter only to sites that want children among their audience.

Those that don't, or don't care, would not seek a rating, and would not be accessible to devices that parents control. Otherwise they would be free to host whatever legal content they wish.

[inigyou]

Twitter wants to serve children and it also wants to serve porn stars, and there's no good reason it can't do both as long as it keeps them separate, but your proposal doesn't allow it to know who's a child so it can't keep them separate and in response to that it has to ban one or the other from the platform entirely.

[pcrh]

Twitter can make a "Kid's Twitter" (Kitter? Kidder?) if it wants. It's far less oppressive than forcing people to reveal their identity to Twitter.

[inigyou]

Great now you have two websites. This seems way more inconvenient than simply hiding over18 tweets if the user is a kid?