TL;DR: You can now roll back the DOB on an Apple account
I’ve been asked about this on many occasions, When someone turns 18 and their account is migrated to an adult account, they no longer can be managed for Family Sharing, Screen Time, content restrictions, filtering, and other parental controls ect..
Apple has now changed this now explicitly documents a process for changing the birthday from an adult age to a child age and converting the existing account into a Child Account
Published Date: September 14, 2026