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.