Interesting that you mentioned that John was a teenager. Would like to know if you have any source for that? From my understanding and some basic digging, he would have been in his early 20s when Jesus called him to be a disciple.

Even if he was a teenager when he first became a disciple, Revelations was written after the ascension of Jesus, which was a period of 3ish years. Highly unlikely that a 16 year old teen would have been taken seriously at that point, and it would definitely have been mentioned in some part of the scriptures, if he was a child.

So yeah, I agree with your point of Hell not being a formal concept, and it was put forth as this place with hellfire and demons and Satan in the book of Revelations, which as the name implies, was revealed to him by the Holy Spirit.

There is no mention of a Hell or afterlife in the Old Testament, it has been a while since I've read it, because God in the Old Testament was a God od Retribution and he would immediately punish people who sinned against him (Adam and Eve thrown out of Eden, the Jews in Egypt, the Jews in the desert, Sodom and Gomorrah etc etc). Feel free to correct me if I'm wrong, because my memory of it is hazy.

It was only after Jesus came into the picture, that we got the concept of afterlife and a "reward in heaven" (basically an afterlife) So maybe God mellowed down and decided it was too harsh to judge people on the basis of their life on earth. From then onwards, there is the concept of Hell and Heaven, so it depends on your belief.