Managing personal Outlook.com account settings, security, and privacy
Hello,
Is it because your current dob is reflecting you as a minor?
If so, it won't let you edit your own dob.
You need to create a new account just for now, make sure this new account has an adult dob.
Then invite your own "child" Microsoft to join the new account's family group. Once joined.
Go to https://account.live.com/mykids.aspx sign in with the new account (adult one), you should see your "child" account listed. Click on "Edit this child's personal info" And it will let you update the date of birth for this child user.
Once updated, you can then use the "Remove consent for this child's account", then go back to the family safety page, click on this child account and choose "remove from family group".