Making a child account into an adult account.

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

The only way for it to change to an adult account is for it to age into one accorsing to the birthdate that you originally submitted

Only a parent using the Microsoft family service can change Xbox privacy and online safety settings for a child account. If you're having trouble changing your child's settings, make sure your Microsoft account is a parent account in a Microsoft family.
See whether you're a child or an adult in a family by signing in at account.microsoft.com/family. To change your child's settings:

• Sign in to the Xbox settings page with the parent’s Microsoft account. You may be asked to verify that you're the account owner.
  If you're having issues signing in to a Microsoft account, go to
  Microsoft account help.
• Choose the gamertag for your child's account. If you want to update the privacy and online safety settings for the parent or adult account, choose that gamertag instead.
  
• Select theXbox One/Windows 10 Online Safety
  tab or the Privacy tab, review the settings currently in place and then update those you want to change.
  
• Select Save before leaving or switching to another tab. If you don't save the changes before you move to the other tab, your changes won't be saved.
  
• Have your child sign in and sign out of their account to make the changes take effect.
  

Get more help managing your Microsoft family.

Hi,

You can try this page: https://support.microsoft.com/en-us...account-age-verification-for-parental-consent

Here you can "submit a proof of age". Of course you need to have the legal age of 18+ (or 20+).

You can't change a child account to an adult account and to make billing available under an adult account you just need to register a method of payment under the adult account