it's a tough question! I think that anyone under the age of 10 is basically joining to play the "fun" games and apps. But anyone over the age of 10 is just keeping in touch with there friends and family. I don't see major issues with anyone of any age joining however, anyone under 16 should have a parent monitoring their posts and their friends list. FB is a great invention!
There is one. You can't sign the EULA unless you are 13. Same with all major email systems. You are required to say your 13 or older. Personally..If you are the admin in your home and you have a good idea on how to set up this services then your fine.
My Son is 8 and has a Facebook, Gmail and Steam Account. Everything is configured in a way where he doesn't have full access to each. Gmail only allows email from family, Facebook is set to private everything and the password reset tool is my account. And he can only use Steam with Visa gift cards.
I think you've got a good set-up there, Jason! I do think that kids should be 13 or older to have independent accounts on Facebook and things, but if their parents set one up for them and make sure to keep track of what's going on then there shouldn't be a problem.