The key difference with the YouTube example is that Google requires you to create an account (which helps them track you) and specify the account's age. They also require proof of ID these days to give you access to age-gated content, which is possible, but quite tricky, to bypass.

The idea with my fallback solution is that it could work completely accountless. Your browser just reads from your operating system what your age is, the same way they can read what screen resolution you have or what version of your browser you're running.

With robust parental controls in place, an OS should be able to prevent a child from installing any software without the parent's consent (by the parent typing in a password that the child should not know). If it's done robustly, the only way a child should be able to get around it is by dual-booting (or live USB-ing) into an entirely separate petition that their parents don't control. And I'mma be honest, any kid who can figure that part out deserves free reign over their computer.