Change birthday from child
How do I change my daughters birthday in her iTunes account? She put a fake one in cause that's what I tell her to do for random accounts... but probably shouldn't have for this one đ
You can't on a child (i.e. under-13 in Family Share) account : Update the birth date associated with your Apple ID - Apple Support
Be sure to enter your correct date of birth. After you change it to any age below 13, you can't edit it. Learn more about Apple IDs for children.
If it's not a child (i.e. under-13) account you can change as per the instructions in that link.