The law was just a way to get around idiocy that removing some problem algorithms was
too hard on social media platform's part. It's a bit like country roads, there's a ban on kids riding or driving vehicles on them without a licence, there's a loose understanding what's not going to attract the law, then there's behaviour and areas where it's a total no go - then the police will work hard to sort that out.
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.