The official age policy, which differs from what Guspasho posted, is in the policies found on the registration page on the main.
The biggest difference is that 13*-19 years olds MUST have a con guardian on site (not necessarily by there side.) This can be an older cousin, yes.
*Yes, last year it said 14. This year, policy says 13.
If you are under 18, I'd suggest you study this part of the policies in particular.