Given that Marcelo in the Real World was an honorable mention for the Amelia Elizabeth Walden Award, judged by the Assembly on Literature for Adolescents, there's no question what genre it belongs to.
The two main characters, Marcelo and Jasmine, are 17 and 19 years old, respectively. Ixtel and Wendell are teenagers, too. The book deals with typical teenage problems—sex, parents, school, summer jobs—even though the narrator is anything but typical. Contrasting Marcelo with other teenagers shows us how different the lives of young adults can be, and isn't learning about other people's lives half the reason we read in the first place?