When signing up you are agreeing you the sites Terms Of Use(http://www.wizards.pro/terms-of-use). Number three tells how you must be at least thirteen to register on this site. A lot of players have ignored this while signing up lately and have ended up banned because of it. We're not banning people you may have become friends with to be mean, but because its not legal for them to be on here.

From the Terms Of Use:
3. Your Responsibilities and Registration Obligations.
In order to use this web site, you must register on our site, agree to provide truthful information when requested, and be at least the age of thirteen (13) or older. When registering, you explicitly agree to our Terms of Use and as may be modified by us from time to time and available here. For more information, please read the Children's Online Privacy Protection Act of 1998 (COPPA).

You can read the COPPA here:
http://www.wizards.pro/childrens-online-privacy-protection-act-of-1998-coppa
or
http://www.coppa.org/