Revealing mistake: After receiving Wendy's real kiss aboard the Jolly Roger Peter quickly flies up and proceeds to fly back down. As he swoops down to the deck and lifts up two swords, most of the kids' eyes are not in sync with Peter's movements. (01:31:45)
Revealing mistake: After having been kissed by Wendy, Peter flies towards Hook, who is tangled up in the rigging of the Jolly Roger and slashes the lines of the rigging thereby releasing Hook. As Peter swipes at the lines it is very apparent that he only touches a few of the pre-cut lines and the rest simply fall away. (01:31:50)
Revealing mistake: Jason Isaac's stunt double is easily seen in quite a few shots, for example when Hook duels with Peter on the Jolly Roger. His hairline is lower and the shape of his beard is different. (01:32:25)
Revealing mistake: After Peter watches the Darlings and the Lost Boys through the nursery window, he begins to fly off. The camera pans onto the rug to show Peter's exiting shadow on the long bright spot of light coming through the window, which Wendy notices. This lit area on the rug is much larger and brighter than in the previous or following shots. (01:41:35)
Answer: It's not that you never age, it's that you never grow up, i.e. pass into puberty and become an adult.
Myridon