I don't really see a problem where there is a standard api (or even syscall!) to rethrieve a persons age bracket and for various apps being able to easily implement it. But please make it fucking optional.

Make it optional and assume an adult otherwise, it's a good idea if it's optional and doesn't have dumb fines, you could have fines for not enforcing it / not using the api [porn sites] that already exists [and it doesn't work since 1 button is not age verification].

I see this as a good way for parents and institutions to set up their phones, school laptops etc and would pretty much solve the large majority of these issues while having a fraction of the invasiveness.