I would like to change my sons child account to an adult one,

Hello, Your childs progress will be saved as a xbox profile store's everybody's progress on Disc games and digital, and he will indeed keep all his data. Changing your child's account to a higher age will not affect his data in any way. Yes he will still be able to be removed from the family sharing even if his account is registered as an adult account.

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Happy Gaming

As already mentioned,you can't change the age on a account.

You should read previous comments before posting.

can he still be removed from the family sharing?

what if he is still a child and i turn it into a higher age?

Sharing is done through the Home settings so if the person who has the Home console set as their Home, removes it, he would lost the ability to access their stuff.

If you're trying to remove him from the family group, then you should still be able to remove him.

You can't change the age of a Xbox account once it's been created.