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.
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.