Safety is and not a problem. They are posers. It is rarely you will find the real person. I say its safe for 11 and over but their not even meant to have an account so I would leave it until they are old enough to have an account.
Once they are older enough to have one, just leave another couple of years until you are sure the responsible enough to have an account. Keep double checking their friends and information though as you never know who they could add. Keep them under your thumbs!