That's not such a bad idea actually, because text messaging is a primary part of predatory "grooming" activity.

I'm sure those predators would appreciate "secret" Facebook messaging. They'll probably tell their victims that nobody can ever see the pictures they send "secretly".

How are you going to check the age of somebody? Requiring them to sign up with an ID? From my experience PayPal for example doesn't actually verify your age, although you have to provide an ID.