how to change an age of a child account
How do I change the birthday of child on my account - the year was entered wrong from the beginning and she is now an adult ?
iPhone 8, iOS 13
If the age is set to something over 13 years old, you can log in to https://appleid.apple.com and update it there.
If it's set to under 13, then you will need to contact Apple support directly to get it changed.
click here ➜ contact Apple Support
More info here ➜ Update the birth date associated with your Apple ID - Apple Support