I agree it would be nice but it requires more investment than my plan and companies really do not want to spend an extra cent for children safety.
Furthermore, OS makers could lobby for additional age gates. I.e. a child phone for ages 15 and under, a teenager phone for ages 16-17, an adult phone for 21.
The system is so complicated that I think many parents simply get confused and do not want to use it.
Instead there should be a single checkbox: "allow safe content only on this device". That's all (and you can leave detailed settings for system administrators).
[1] https://www.internetmatters.org/parental-controls/smartphone...