I would point to two events that have the most to do with John's coming of age.
First, there is the event that John refers to that happened in his childhood. This was when he was willing and able to wait for his father in the doorway of the dead place and to hold the metal his father brought out. It was then that he knew he was special, beginning his coming of age.
Second, of course, is John's time in the apartment in the ruins of New York City. It was while he was there that he came to truly understand what the Place of the Gods was. He was, for the first time, able to understand the truth about the world in which he lived. This is, perhaps, the ultimate coming of age; the ability to see and to deal with the truth about the world.