Permission for child

Does the account have an Xbox Live subscription on it? Or is another account sharing Gold?

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.

Ya

That's why we asked if you had an child account

Child accoujlnt make purchases of m rated content without a console or parental permissions

Xbox one permissions don't count for xbox 360 games

Check the permissions of the account if its a child one

Could be it is a child account without permission to play online