The Age of Innocence is a novel that falls into the category of Modernism. This is because Wharton is playing with typical narrative constraints. While, in some ways, The Age of Innocence may seem like a romance novel—the love triangle between Archer, May, and Ellen is, after all, the driving force of the book—Wharton is also playing with the typical tropes and literary conventions of this type of novel. For example, no one gets a happy ending in this story. Ellen ends up leaving New York without ever having a chance to have a relationship with Archer, May ends up in an unhappy marriage with Archer before eventually dying, and Archer ends up pining for Ellen for 26 years, ultimately deciding not to see her at the end of the novel when he’s given the chance. This kind of unsatisfactory ending is typical of Modernist literature.
This novel is also a novel of manners, meaning that the book pays close attention to the social dynamics and “manners” of people of a certain (middle- to upper-class) society. While this type of novel is most often associated with authors like Jane Austen (who did stick to the typical romance novel tropes), Wharton is penning her own American version. The Age of Innocence contains many of the classic elements of novels of manners: characters acting polite and even-keeled while the narrator reveals they are actually feeling immensely emotional or out-of-sorts, misunderstandings between characters because of a lack of direct communication, and judgment of social outcasts (like Ellen) who don’t follow the agreed-upon rules.
Finally, The Age of Innocence can be considered a work of historical fiction because Wharton published it in 1920 and it is set 50 years earlier in the 1870s. She did not arbitrarily choose this period but did so in order to illuminate the differences between the New York society of the 1870s and of her present—much progressive change had happened as well as the devastation of World War I, and she was juxtaposing the "age of innocence" with both the war-torn and progressive-minded energy of her time.