Isn’t that because of COPPA, which makes it illegal to retain any personally identifying information about or from people under age 13?
For instance I’m directly mentioning Youtube, but outside of comments it’s still somewhat usable for a 13- yo kid.
Twitter could have set a mode for 13yo (idk, have parents pre-vet the tweets if they are really into controlling ?) but it looks like it was simpler for them to just ban pre-teen accounts.
I’m not sure facebook can deal with pre-13 yo either.
Basically the shittier the company, the less it will be able to handle the pre 13yo case, and I wouldn’t put the blame on COPPA.