Coming of age in its essence is extremely convoluted. What makes one person grown, can make another childish. Categorizing age is like categorizing gender. Yes you have a set age, just as you have a physiologically set sex, but that doesn't necessarily mean that internally you assimilate that. Inversely, you may not portray that to the world.
Stephen from the beginning has been extremely hard to pinpoint in age. The way he makes decisions sets his age at one point, but the internal dialogue that the reader is always tuned into ages him quite a bit. I would argue that pretty much from the beginning he himself is not consciously making decisions. He has always sought out advice from other people whether by following all of their previous advice or looking to his peers to raise his bravery. Even when he is alone (such as the end of chapter two with the sexual experience) he is driven by his body not his own mind. I think that this widespread trait has challenge the maturity that he has displayed in other traits.
In chapter three we learn that he is 16, which shocks me. Yes, we talked about age in class, but this was an affirmation of how young he was. The Stephen that the reader has been let inside of acts so much older, his speak of hell and life as a whole to me adds years of deep sadness and discontent, something I struggle to see in a 16 year old boy.
Due to conflicting traits I cant really make a prediction about when Stephen will "come of age". However, at this rate I don't think it will be too long. Joyce has a way of packing a large chunk of his life (but maybe only portrayed by one scene) into a small section. I'm excited to see where this goes because I can relate to a lot of the small details in growing up. Small things like the arrogant nature of everything you do, having trouble seeking out comradery upon arrival at a new school, sometimes even the discrepancy between your outward appearance versus what is really going on inside.