I can't change the birth date for a child's Apple ID

I just added a my Apple ID account to my mom's family group (I'm the kid) and I'm trying to change the birth date from 14 years old to 11 (since I made this account when I got my laptop and didn't know what family sharing was). When I try to do that, there's a popup that says 'Could not connect to iCloud. You are not eligible to create/edit an account at this time.' How do I fix that?