I'm trying to enroll for the Apple Developer program (as an individual) but it seems I just can't.
I AM 13+ (I'm 15-years-old, specifically), but it just won't let me enroll.
You have to be at least 18 years old to enrol in the Apple Developer Program yourself. There however, I believe, exists a way for you to enrol otherwise. If I am not mistaken, you can enrol using an Apple ID of someone who is over the age of 18 (parent/guardian, for example) and use their account to access the services that the Apple Developer Program provides. I would recommend double checking with Apple Developer Support before continuing.