Online giant Reddit and advocacy group the Digital Freedom Project argue the ban is unconstitutional because it excludes young people from civic engagement.
As for the under 16 ban, it's more that social media platforms snubbed the regulators reacting to a consensus of concerns over bad algorithms, and slowly esafety via govt regulations found another way to skin the cat. Kids that are emotionally strong and robust will most likely skirt the ban, those who are susceptible that the platforms used to target purposely ... well those platforms no longer have any legal legs to stand on if things go wrong.
You can't restrict a portion of the population from doing something without identifying everyone, and that's probably the actual goal.
I don't know if there's any more intention than that, though almost immediately with the ban I was enjoying not seeing the latest 13 to 16 year old twit posting their recent car stealing / wild ride around the streets the tv news services had found on some social media page to play up youth crimes.
If the social media that could not remove the purposely age targeting algorithms parents were up in arms about in 2018 .. 2019, haven't already done so since the under 16 ban came in, to put it bluntly they're not paying decent capable lawyers.
As for proof of age, esafety already proposed how that should happen, arms length and not direct proof with personal identification directly with the platform. It's social media platforms wanting photo id or other PII ... but of course that's exactly what those platforms want, more PII.