The Red Pony by John Steinbeck tells of a young boy's entrance into manhood. Throughout the novel, Jody faces numerous conflicts and disappointments that chip away at his youth.
One event which causes Jody to grow up is his ownership of a red pony. After getting the pony, Jody asks his father if he can take the pony to school to show his classmates. His father does not believe it to be a good idea, because the pony is not "halter broke yet." His father comes up with an alternative: bring his friends home with him after school. Jody grows up because of the pony in two ways.
First, Jody is completely responsible for the pony. He is warned that if he does not take care of the pony, his father "will sell him off in a minute." This tends to be the first step towards adult responsibility—taking care of another living being.
Second, Jody's friends feel that Jody has grown up when they see his pony.
Before today, Jody had been a boy, dressed in overalls and a blue shirt—quieter than most, even suspected of being a little cowardly. And now he was different. They knew instinctively that a man on a horse is spiritually as well as physically bigger than a man on foot. They knew Jody had been miraculously lifted out of equality with them, and had been placed over them.
Here, Jody has become a man in the eyes of his peers. So while this was not necessarily an event which caused him to grow up per se, the responsibility of taking care of the pony led to his friends identifying him as a man. The "growing up" happened as he showed his newfound responsibility off to his friends.