Xbox Account Not Adult

In order to create a child account xbox requires an adult account. Children under 13 are not allowed to create accounts without an adult account. Have the adult user purchase the game or change the parental controls.

create another adult account on your xbox and use that to change your current account to adult as well

Hey there!

You need to have an adult profile to change your Gamerpic. If you have an adult account you will need to provide a valid credit card on your account and make sure you're birthdate is 18. If your account was created by an adult account and are in a Xbox Family
only the adult account can promote you.

That means you have a child account and an adult account on your Xbox. And you can only change these settings on the adult / parent account.

I found this, hope it helps.

Promote a child account to an adult account in Xbox

Xbox LIVE determines whether an account is a child account or an adult account based on the age of the account owner. Account owners who are younger than 18 are given child accounts in the United States. The legal adult age varies by country and region (for
example, the adult age in Korea is 20).

Child accounts are tied to an adult's Microsoft account. The adult account can control Family Settings and restrict some features of Xbox LIVE.

After you reach the age of adulthood (which varies by country and region), you can promote a child account to an adult account. This promotion removes the Family Settings and Xbox LIVE restrictions.

To promote a child account to an adult account

Sign in to Xbox LIVE by using the account that you want to change.

Go to Settings, and then select Account.

Select Promote Child Account.

Note The age of an account owner is calculated by using the date of birth provided when the account was created. This date cannot be changed. If you entered an incorrect date of birth when you created the account, your only option is to create a new account
using the correct date of birth. This option is available only if the date of birth for the account indicates that you have reached the age of adulthood (18 in the United States). The legal adult age varies by country and region.

Follow the on-screen instructions to promote the account.