Child Apple ID
If I set up a child Apple ID to use in my family group, later when he is older than 18 years, can he later use the same Apple ID independently as his own account, without being in the family group?
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Yes.
The child account will be able to leave the family group when it is a teen account.
when it is over 18 it will be a completely normal account.
The age depends by country
Family Sharing and Apple ID for your child - Apple Support
Remove a child from your family group
Your child has to remain part of your family group until they turn 13.* However, if necessary, you can transfer a child to a different family group.
If you need to see the options you have for managing your child's account, you can sign in with their Apple ID and password at privacy.apple.com.
* Age varies by country or region. Under 14 in Austria, Bulgaria, Cyprus, Italy, Lithuania, and Spain. Under 15 in Czech Republic, France, and Greece. Under 16 in Croatia, Germany, Hungary, Ireland, Kosovo, Liechtenstein, Luxembourg, Netherlands, Poland, Portugal, Romania, Singapore, Slovakia, and Slovenia. Under 13 in all other countries.