a) Content controls don't work, what are the government thinking? b) This is parents' problem, they should use content controls.
Individual action doesn't work because it only takes one kid in the class who doesn't have parental controls then everyone loses. There's also obvious workarounds such as VPNs and a teenager walking into a pawn shop with £50 for a second hand smartphone without parental controls.
It also makes no sense that parents can't be bothered to turn on parental controls yet can be bothered to run a national grassroots campaign for this stuff (see e.g. http://smartphonefreechildhood.org)
See also- I Had a Helicopter Mom. I Found Pornhub Anyway: https://www.thefp.com/p/why-are-our-fourth-graders-on-pornhu... 8-year old watches violent porn on friend’s iPad: https://www.thesun.co.uk/fabulous/32857335/son-watched-viole...
Although your idea of an OS-level age flag is also being pushed by the Anxious Generation's Jonathan Haidt, so definitely has merit/traction as an alternative.