Adult Account No Longer Available...How to Free Child Account?

WHY SHOULD HE BE PAYING IN THE FIRST PLACE YOU GREEDY *calms* oooook

if you used the correct birthdate, it should become an adult automatically. Otherwise, you can promote the child account to adult by adding a payment instrument and a billing information to your Microsoft Account.

you're parent's may still be able to recover their account via the secret questions etc, even if the old email is invalid.

The steps you're looking for to promote a child account can be found here. There is also a

page for age verification, which has a few methods to verify your age as a non-child account to allow you to change your Privacy and Online Safety Settings.

Also, just to be sure, you should double check your birthday for correctness at
your Microsoft Account page
under Your Info.

...I had to sign into my old account, which is still considered a "child" account (even though I am over 18 now).

...

Is there any way to remove the child account without having to sign into the parent account? Alternatively, is there any way to change the privacy settings or log into the account in question to make the changes?

The age of your account is determined by the birthdate you entered when you created your account.

If according to that birthdate your account is 18 years old, it'll automatically become an adult account.

While you can try to see and change your birthdate at https://account.microsoft.com/profile/ it will not change the birthdate of your Xbox profile. So you will still be
stuck with needing an adult parent account.

If you do have an adult account, you can try to sign in to that adult account, and go to
https://account.microsoft.com/family and add your child account to that account's family.

Or if the account is incorrectly classified as a child account, try to
verify the incorrect child account to be an adult account according to the instructions here.