Because Jim is looking back at the events of his youth from the perspective of an adult, we have two different Jims to think about. We have Jim-the-narrator, the wise adult, and Jim-the-character, the protagonist who grows up in the course of the novel. Many of Jim's reflections and musings belong not to the young boy of the novel, but rather to the grown adult who looks back on his youth. His insights on the social order in the town of Black Hawk, his understanding of the perseverance that drives the hired girls, and his musings on his feelings for Ántonia all fall under this category.
But we have to remember that, while Jim narrates the "memoir" about Ántonia, the novel has a second narrator, whom we meet in the introduction. We don't know too much about this narrator – no name or gender or age – except that he/she is a writer and also knew Ántonia as a child. One way to interpret the introduction is to imagine that the "narrator" is Cather herself. This makes sense when you consider the biographical connections between the events, settings, and characters of the novel with those of Cather's own childhood in Red Cloud, Nebraska.