Can't change my privacy settings

This is because you're underage or used the wrong age when setting up the account, Unfortunately once an age is set it's permanent. While the system will let you change the age anytime, It only recognizes the age you used when you set up the account. This
is to prevent kids from just changing their ages to bypass things. You will need your parents help or you will have to send in an age change request form and see what they say.

in the following link shows you how to do it from pc and xbox https://privacy.microsoft.com/en-us/xbox-one-privacy-and-online-safety

I have had my account for ~8 years, but I still do think it's odd that back then I could play multiplayer no problem, but NOW I can't.

Hi! My name is Pam. I'm an Independent Advisor and I'll be glad to help you today.

If your account is underage you will need to be in a family group as a member and an adult account will be able to change the privacy settings of your account. I suggest to ask an adult to add you so they can manage your settings.

I hope this helps. Feel free to ask back any questions and keep me posted.

That's real weird. I would recommend you take a look at your age for the account, and if you need to

perform Age Verification, after making sure that it's not listed under another "adult" account as a child.