How does this change in Gene reflect the theme of “Coming-of-Age”?

Gene tells of how they were children of "careless peace," set apart from adults by their lack of knowledge of the war, and their utter abandon to their own small, happy worlds. Lackadaisical activities of the happy, peace-enveloped juniors are juxtaposed with the semi-military drills that the seniors have to endure. Just as the war encroaches upon the boys at school, their adulthood also looms before them; Gene feels this especially, and this is one of the things that traumatizes Leper‹being suddenly thrown into the world of adulthood. Throughout the novel, Gene notes the difference between his state 15 years after Devon, and his state while at the school; he notices differences between the way he is and the way he was, and how age has changed him all in all.