We should be to change it since a lot of players didn’t add a birthday or just did a default without realizing this. My EA account has my correct date and that should be the default date for all EA games.
Support will tell you that you need to submit an official/personal document that shows your birthdate to change. I find this a bit too much for an online game, I don’t feel comfortable submitting that type of information for a game. It shouldn’t be that hard. It’s not like I’m accessing a bank account or something. They can see my birthday on my EA account and change it on Galaxy of Heroes without issues.