Wendy's exact age is never revealed, though we do have some clues that help us determine her approximate age. We know that Wendy still sleeps in the "nursery" with her brothers, so she is still considered a child by her parents' standards: thus, she is likely younger than thirteen or so. Yet she is also old enough that her sense of responsibility is developing. It is Wendy who attaches Peter Pan's shadow before they even leave for Neverland. In fact, she laments that she should have ironed it before sewing it back on, demonstrating a more experienced eye for details than would be possessed by a young child.

When Mrs. Darling asks Wendy about Peter Pan early in the story, Wendy comments that he is "just [her] size," indicating that she is in an older stage of childhood.

In Neverland, she assumes the role of a mother, ensuring that the children are clean and fed. When Peter Pan tells Wendy that in Neverland, they need a "nice motherly person," Wendy replies, "I feel that is exactly what I am." She then spends "many joyous evenings" with the children, "tuck[ing] them up in the great bed in the home under the trees."

Wendy is technically still a child by society's standards, but her ability to provide care for younger children and her physical height likely indicate that she is somewhere around twelve or thirteen years old.