I think Facebook and Google have TOS agreements that state a user has to be above the age of 13/14. If you stated your age on Google+ as younger, Google disabled your account.
Those blocks are really unhelpful for everyone but the sites (who get to pretend they don't have children as users) for that reason - they don't stop under 13's from registering, but they do ensure there's no remotely reliable data on who are under 13.