Showing adult content to minors is also probably not an insignificant part of their business (certaintly a major part if the classification of social media as adult becomes more widespread), and having age be an os-user property might give children more opportunity to subvert the verification. And if enough applications end up behind the maturity wall, they can count on children to badger their parents into setting their account to adult, and the industry will absolve itself of all responsibility once more.