Change account to child

You can change a child account to an adult account, but you can't change an adult account to a child account. In order to manage them, you would have to create new child accounts for them.

Only an adult (parental account) can make the changes in privacy settings for a child account. The child account can't perform the changes itself.

Some xbox live features need changes to be made in the privacy settings of a child account to be used. The changes in the child account can be done through the parental account as shown here: Change
your child’s Xbox privacy and online safety settings

You can't exactly change a child account to an adult account until the child has actually turned 18. However, you can change their permissions. You need to log into the adult account, and under family settings, you can change the permissions of your child's
account.

Once you register an xbox live account as a child account you can't change it. The parental account can change privacy settings of the child account though.