So? The same problem exists for having the OS broadcast the user's age range to all apps/services/websites: the service outside your jurisdiction doesn't have to actually restrict content based on age.

At least with the reverse system (services broadcast an age rating), you have some nice properties:

1. You can set it up so that if the service doesn't broadcast an age rating, access is denied.

2. You aren't leaking age information (even if it's just a range) to random websites outside your jurisdiction.