Changing date of birth on account

See Update the birth date associated with your Apple ID https://support.apple.com/en-us/HT204164

To change a birth date to any age below 13,* you must be a member of a Family Sharing group, and your family organizer must approve the change. After you change a birth date to any age below 13, you can't edit it.* Learn more about Family Sharing. 

If it is set to below 13 yes old, contact Apple ID Support to see if they will reset it. I have read a couple of posts where Support did that. 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.

If the child is over 12, change in their Apple ID Account https://appleid.apple.com/#!&page=signin