The only time Peter truly grows up is in Sir James Matthew Barrie's 1911 novel, "Peter and Wendy." In this book, Peter returns to London to help Wendy's daughter, Jane, who gets kidnapped by Captain Hook. During his time in London, Peter interacts with the human world and begins to feel an attachment to Jane and her family. This leads him to start questioning his eternal childhood and contemplating the idea of growing up. However, in the end, he chooses to return to Neverland, unable to fully let go of his magical life.
In some reinterpretations of the Peter Pan story, such as the 2003 movie "Peter Pan," the character eventually ages due to unique circumstances or through external forces. Still, the essence of Peter Pan as an eternal child remains a central theme throughout various adaptations and representations of the character.