Changing Child to an Adult

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.

Is the account in question a child's or adult's account? If its a child account you may want to change it to an adult account and give it a shot.

The adult account need to upgrade to adult or wait until the child account becomes adult in prder to change it...

I can't remember 100% but I am sure whoever set up the child account can change it. Like if you made an adult account on the xbox before making the child account that adult account can change wether the child account is a child account or and adult one.
I will just test this out myself now and post how it went

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.