They aren't. The device administrators (ie. the parents) set an age range in the OS' config and then the OS tells applications the value. That's it.

In some jurisdictions/countries they have to call some government service which then tells them whether a person is above or below a certain age.

Theoretically you could make this double blind and that's the story they are telling.

We have no way to (dis)prove it though.

Alas, here goes the freedom, with thunderous applause.