This is an interesting question because maturity isn't really a theme that jumps to mind when it comes to this novel. That said though, I would say that Nick is the only character who matures. At the start of the novel Nick comes from the Midwest to the East in order to learn the bond business and make it rich in New York. He is able to rub elbows with his rich cousin and all her wealthy and elite friends like Jordan Baker. He clearly has positioned himself to achieve his dreams of success and wealth. As the novel progresses, Nick recognizes the deep flaws in all of the people that he thought he would admire. Jordan has possibly cheated at golf; Tom has a mistress and is rude to those beneath him socially, like Wilson; Daisy is a bit flaky, a cheater, and kills Myrtle without ever taking responsibility for her actions; Gatsby gains his wealth from illegal activities and is foolishly loyal to another man's wife, Daisy. There is very little to admire in any of these people. By the end of the novel he recognizes the carelessness of these people, and he matures to an understanding that what he thought these people represented (wealth, status, sophistication) is actually a shallow representation of the American Dream. His return to the more grounded roots of his Midwest youth show a sense of maturity for Nick. None of the others characters change at all through the course of the novel.