Boo Radley, though an adult in years, is much like a child. He is still living with family despite his adulthood, and he is still serving his "restriction" imposed on him by his father for the juvenile pranks in which he was involved. He is sequestered in the house and does not work outside the home. He seeks the company of children, Scout and Jem, in a "hide and seek" fashion. In short, he is not acting as an adult or "of age".
He does achieve this status when he rescues Scout and Jem from Bob Ewell's knife attack. He must leave his childhood home and confront an adult situation as an adult. When he is present at Jem's bedside, is is now of this world. His conversation with Scout on the way home confirms this "coming of age".