Peter Pan appears in various works by J. M. Barrie, and the way in which he is presented is not entirely consistent. None of the sources directly mentions his age, but in the 1911 novel Peter and Wendy, chapter 1 ends with the following description:

He was a lovely boy, clad in skeleton leaves and the juices that ooze out of trees but the most entrancing thing about him was that he had all his first teeth. When he saw she was a grown-up, he gnashed the little pearls at her.

Children typically lose their first set of teeth between the ages of six and twelve. However, a complete set of first teeth is rare after the age of seven. Elsewhere in the novels and in the play, Peter appears to be older than this and is generally portrayed on stage and in illustrations as being closer to the age of twelve.

Another way of answering the question would be to say that, since Peter Pan is not real—or, for that matter, a realistic figure—he need not have a real age. Presumably there must be a set amount of time between his birth and any given point at which he appears, but by these standards, he could easily be an adult. Given that the Lost Boys grow up, it is clear that merely being in Neverland does not halt the aging process. This is a magical phenomenon which is peculiar to Peter, and, since the author never explains it, the reader can do so as she or he wishes.