Change Age for Child Apple ID
How can change my child’s Apple ID birthday date
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Read the first link please:
Update the birth date associated with your Apple ID - Apple Support - https://support.apple.com/HT204164 - "Be sure to enter your correct date of birth. After you change it to any age below 13, you can't edit it." In that case contact Apple:
iTunes [Store] Support - Use this form for assistance with account and billing, code redemption, or purchase questions --> https://www.apple.com/emea/support/itunes/contact.html
Contact Apple for support and service - "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232