That said, the problem is that sites don’t know for sure who is over and who is under 13. And more robust age proofs have lots of problems: https://www.techdirt.com/2022/06/29/california-legislators-s...
Even if they did, many parents strongly disagree about what content is age-appropriate. When I was growing up my mom was shocked that other 10yo were allowed to view James Bond films.
So while you might be able to set some baselines, parents still need to do a lot of the legwork to make sure kids are ingesting media that’s aligned with their values. This was true on older media and it’s still true today.