Birth date
How I can change birth date in iPhone of my sister( 10 years)?
iPhone 8 Plus, iOS 13
You can't. Note the under 13 years old restriction in the article below.
Apple ID - Update the birth date associated with.
Contact Apple ID Support for assistance. See the Apple Support articles below for contact information.
Apple ID - Official Apple Support.
Contact Apple for help with Apple ID account security - Apple Support. (Link doesn’t always load)
Get Support Work through the screens to get a chat option. You may have to pick a slightly different problem.
Apple Support schedule phone call
https://twitter.com/AppleSupport. People have posted it may take an hour or more to get a response.