how do i enable minecraft multiplayer on java edition?

I totally understand and we need to check why you are still getting the same error even if you enable the settings on the account. Kindly check the date of birth that you can see here, and did you recently change this?

https://account.microsoft.com/profile

Hi, modog7. I hope you're doing well and safe out there. I'm an Independent Advisor and I’m happy to help you today.

Sorry to hear about your issue with your Multiplayer features. Let's try some steps that might help. Kindly sign in to https://account.xbox.com/settings and make sure that all multiplayer settings are set to “allow” or “everyone”:

Please click “submit”, once confirmed restart your device to re-sync and make sure all changes are saved then Kindly open the Microsoft Store, then click on the profile picture in the top right corner. A pop-up box will appear allowing you to sign out then once signed out, kindly sign in again using the account then try again.

If the issue persists, I suggest you try the steps here: https://help.minecraft.net/hc/en-us/articles/44... or contact Minecraft team as advised on the link.

does it need a certain age to allow multiplayer?

Also no i did not recently change the age.

The problem isn't that i can't enable the multiplayer in family thing, it's that after i enabled multiplayer it still wouldn't allow me to play multiplayer.

Yes, you need to be an adult for this to work. If you change your date of birth, then this will not work.