Can facial analysis technology create a child-safe internet?

Suppose you pulled out your phone this morning to post a pic to your favourite social network – let's call it Twinstabooktok – and were asked for a selfie before you could log on. The picture you submitted wouldn't be sent anywhere, the service assured you: instead, it would use state-of-the-art machine-learning techniques to work out your age. In all likelihood, once you've submitted the scan, you can continue on your merry way. If the service guessed wrong, you could appeal, though that might take a bit longer. The social network would be able to know that you were an adult user and provide you with an experience largely free of parental controls and paternalist moderation, while children who tried to sign up would be given a restricted version of the same experience.