There is also the problem of mission creep. Once the infrastructure is in place, to control access to age-restricted content, other services might become out of reach. In particular, anonymous usage of online forums might no longer be possible.
OS-level ability to verify the age of the person using it absolutely provides infrastructure for the OS to verify all sorts of other things. Citizenship, identity, you name it. When it's at the OS level there's no way to do anything privately on that machine ever again.