Family sharing
How to make your child on family sharing switch from adult to child?
iPhone X, iOS 12
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Child accounts are under-13s, if he/she is under-13 and you put the wrong date of birth on the account log into the account and change it (if you change it to under-13 you won't be able to change it again until they reach 13) : Update the birth date associated with your Apple ID - Apple Support