I would describe John as young. He refers to himself early in the story as the son of a priest. He thinks of himself, first, as somebody's son. I believe that makes John fairly young. I would guess under the age of 20. I also would describe John as physically strong. He lives in a world that requires people to hunt and forage for food. John is likely physically active a lot of the time. We also know that he uses a bow and arrow to shoot and kill a panther. That's an impressive show of strength.
John is also emotionally and mentally strong. That also makes him brave. John knows that it is forbidden to go east, yet he feels the need to discover what is out there. That mental fortitude (or stubbornness) is what allows John to bring himself to defy his father's explicit reminder about not going east. That's not the only time that John's will overcomes his fears. When he is sitting on the banks of the Hudson, he wants to go home, but he knows that he must continue and go into the city.
John is naturally curious. That is a big part of why he goes east and forces himself to go into the city. I also believe that natural curiosity is why John decides that his people will go back to the city to continue learning. Lastly, John is also intelligent enough to know that furthering his people's knowledge will be beneficial to them.
In John, Benet creates a character who is both brave and cautious. He is full of curiosity for the "Place of the Gods," but also heeds the traditions of his elders and the wisdom of his priesthood. He respects his father's advice but also understands that he must chart his own path and discover what his powerful dreams mean. One defining moment for John is when he kills the panther and takes it as an omen to continue his journey; John has great personal strength and determination to see his journey through to fruition.
Although he may come across to the reader as naive due to his lack of understanding of the world around him, John's fresh perspective challenges the reader to reconsider their understanding of modern technology and society.