Although Philippe Ariès and the historians who followed him have made us familiar with the “invention of childhood,” we may still think of adulthood as a natural category. But that is not the case. Rather, around the turn of the nineteenth century, adulthood becomes the object of the kind of specialized cultural attention that childhood had become over a century earlier. One place where we can see this change happen is the work of Henry James. The adult is not a fact of nature that James exploits but rather an idea that he helps to construct, both through the way he talks about the people who read his novels and through the way he constructs characters within them. As twenty-first-century readers, we may assume that there have always been adults-only books, and that we recognize them when we see them because they foreground things like sex and formal complexity while minimizing things like didacticism. This article questions this assumption both through looking at some of James's arguments for age-leveling fiction for adults and through considering some representations of adulthood in his own fiction. Reading an “adult” novel such as What Maisie Knew in the context of works by contemporary authors such as Frances Hodgson Burnett, who did not write for an age-leveled audience, reveals how Henry James constructs a notion of what it means to be an adult and, by extension, what it means to write for adults. In his work, “the adult” is constituted, counterintuitively, through the gaze of the child.