Why will my account not change to adult?

This is the best answer.

Only if the user is on a Xbox 360. Otherwise the user can't follow the instructions, as the Microsoft Account becomes an adult account automatically on any other platform besides Xbox 360.

Thank you for your help!

-Yager

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 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 Kingdom). The legal adult age varies by country and region. 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.
  
• Follow the on-screen instructions to promote the account.

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 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 Kingdom). The legal adult age varies by country and region. 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.
  
• Follow the on-screen instructions to promote the account.

This is the best answer.

Be aware that the instructions AridLizard provided are ONLY for Xbox 360 consoles.

For Xbox One, Windows 10 and online usage of your Microsoft Account, your account automatically becomes an adult account, as soon as your account reaches legal adult age according to the birth date you entered during creation of the account.

If you sign in on
https://account.microsoft.com and check your personal information, you should see your date of birth.

While you can change it on that website, this does NOT affect your Xbox Account (which will stay on the original birth date and can't be changed). For your Xbox Account to reach adulthood, you'll have to wait until it reaches legal adult age by itself from
the date of birth you originally entered.