Prince Caspian's take on the coming of age theme is totally odd. At first glance, the novel seems to promote a Geoffrey the Giraffe philosophy—i.e. "I don't wanna grow up; I'm a Toys"R"Us kid." Characters like Peter, Trumpkin, and Susan—especially Susan—are criticized for their more grown-up traits, and at the end, Peter and Susan are flat out told they can't return to Narnia because they're too old.
On the other hand, Prince Caspian also requires its characters to be exceptionally adult in many situations. They have to fight in wars, butcher a bear for survival, and Peter even uses politics to his advantage in the fight against Miraz. And let's not forget the killing. In a "having your cake and eating it too" maneuver, the novel suggests we need to stay children and grow up at the same time.
C.S. Lewis once said, "When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up" (source). It's what we think he had in mind when conceiving of Prince Caspian's coming of age theme.
Some characters who are already full grown, such as Trumpkin, come of age during the course of the story.