We've historically relied on Google and Facebook logins for account creation. As of Q1 2012, we launched our own authentication system which opened the doors to users who didn't have Google or Facebook accounts, though our ToS still required them to be over 13 years old.
As of this writing, that means there is no way for users who are under 13 to have legitimate accounts on our site, except by way of Google Apps for Education, a program by Google that schools can opt into with the consent of the parents of the student body to create Google accounts for their students.
We developed a way to create child accounts that are managed by normal accounts. Any non-child account (with a verified e-mail) can create a child account, and manage it. Details are available in: