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.
There were many awful stories of parents who'd created email accounts for their children at birth, and filled them with years of memories and photos, only for Google to lock the account without recourse when they later discovered an age inconsistency. When I worked at Google, I asked the product lead for Gmail why we took such a Draconian step, and they expressed regret but said the COPPA law left them with very little room to maneuver.
Which is of course absolute nonsense from technical point of view. Google could have just provided a zipped archive of the entire account to a parent or legal guardian after verification of ID. But that means work and work which doesn't bring in any money, so why do it at all.
You can have younger -- you just can't manager your own Google account and another "parent" account must be in charge of it. So like removing your own admin/permissions abilities basically.
My son is 12, but has a bunch of accounts with the wrong birth year because we were fine with him having them (if anything he's overly cautious) and so many sites have opted to just ban under 13 accounts rather than put in place proper solutions. He learnt years ago to never give his real birth year because of that and/or to clear cookies and try again if a site blocked him from registering with his real birth year and refused to let him go back and change it.
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.
I'd hoped there was a system in place where Google only locks down the account until the user turns 14 (with the birth date info kept), but... they just locked them down? Man.