My phone thinks I’m underaged

Check to see if you have the correct date of birth set up. Go to Settings > [your name], then tap (Name, Phone Numbers, Email). You should see a birthdate option there. Update the date of birth associated with your Apple ID - Apple Support (NZ)

I'd suggest you ask your parents to use their date of birth since being over the age of 18 may be a requirement. If it still says you're underage then it might be a bug and you should try to restart the device and make sure you're on the latest update.

Well then that means you must be 18 or older to use it, if it works with 2004.

Yes that quite clearly means 18+ problem solved.