Becoming a more mature individual is most evident through the character of Ender, but you could use just about any of the main characters. The story is about young children being trained in battle school to fight against the buggers. Many of the children are under the age of 12, and they are being asked to train for the annihilation of another species. That's huge. If these kids weren't in battle school, they would probably be shooting hoops on the playgrounds of Earth. They are being taught tactics and teamwork, but more importantly, they are being taught to be ruthless. Ender is no exception. The commanders are counting on him to be able to strategically throw away the lives of his friends in order to win at any cost. That's a major coming of age item.
Back on Earth, Card shows the coming of age journey through Peter and Valentine Wiggin. Together, the two of them turn into world leaders in their own right through the writings of Demosthenes and Locke.