Maturity in "A Seperate Peace"

The book A Separate Peace by John Knowles is about a group of students at Devon, a boarding school in New England, going through a school year together. As the book continues, the boys seem to mature more or less throughout the book, sometimes getting mentally older, or sometimes getting mentally younger, varying between the characters.
Phineas says to Gene, “Let’s go jump in the river.” To me, this seems childish. Jumping out of a tree into a river seems fun, but is something just about only a young child would do. What the reader thinks may be entirely different though, every person has their own thoughts about who does what when. But the point is that this does not seem like something a teenage boy at a boarding school would do.
In chapter two, Phineas says to Gene that they should make a club. A club is not something you do in high school; it’s something you do in elementary school. Clubs are a way to waste time when you are young and a way to have fun with your friends when you are a little child. When you are in high school, it is not what you do to have fun with your friends.
When Finny comes up with the idea of blitzball, he is again thinking like a child. The way he comes up with it makes him seem like the six-year-old Calvin from Calvin & Hobbes. He makes up the rules as they play the game, sometimes making it impossible for other people to get the ball.
“Not say anything about it! When you broke the school record!” We all used to do this as children, when somebody doesn’t want something shared, you’d yell it out. It was something you did when you were a young child, not a young adult. When Gene says this, he is not trying to be childish, but he is trying to be funny.
When the boys are shoveling the railroad tracks for money to help the trains pass along on the Boston and Maine line, they seem to have matured more. They are not only doing this for the money, but also to help the country out in a time of need. By doing this, they are not only doing this for themselves, but for the country and their self-satisfaction
When Gene has decided to join the Navy; Brinker, the Coast Guard.