Can a child Apple ID be changed into a adult Apple ID?
If you change the date of birth can a child ID change into a adult ID?
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The only thing that determines an Apple ID as a child or as an adult is the age. If the age is less than 13, then it cannot be changed. If its 13 or over then it can be changed: Update the birth date associated with your Apple ID - Apple Support
However, having an adult aged Apple ID does not in and of itself grant any permissions or remove restrictions on its own.
If the Apple ID has limitations like Ask to buy, then those still need to be manually turned off, and they can be turned off regardless of the age of the Apple ID.
Request and make purchases with Ask to Buy – Apple Support
Age based content restrictions are per-device, and are not related to the age of the Apple ID.