Does self-defense still apply if your assailant is a minor?

Yes, self-defense still applies to the extent you have a REASONABLE fear of injury from their attack and you use only "necessary and reasonable force" to stop them. If shouting, "HEY! Stop that" is all that would be necessary, that's all you would be legally allowed to use.

I once had a 9-year-old pull a knife on me. I simply asked him if he wanted me to break his arm while cramming it up his asss. He put it away and he and his accomplice walked away. I followed him to his friend's house, rang the bell and gave my story to the adult who answered the door. She said "Oh, REALLY? Thanks." I later heard sounds of someone getting quite a spanking.

BTW, pepper spray may not work on someone "hopped up" on anything. If they happen to have a pistol, you may find yourself immediately shot 14 times. Nice going. If you're going to use "self-defense", you'd better be ready for the inevitable escalation.

Yes.

Doesn't matter if it is a minor, because self-defense is usually defined as minimal force to get away. It's NOT fighting back, although yes pepper spray or a punch to the gut would be ok(punching the face would likely be assault as yeah that is more about pain then trying to get away usually, punching the stomach will knock the wind out of them so that's the difference).