How about adding a small feature for whenever someone adds their year of birth when signing in..
If they add 2001 Or earlier as their birth year, A note would pop up next to it which might say "Are you sure this is your Year Of Birth? - According to the Community Guidelines, You have to be at least 13 Or over to join, If you continue to join stating your age as 12 Or under, The Staff could possibly temporarily close your account until you reach the required age." Including a link to the FAQ, That would save a lot of potentially under aged people from joining.