Deliberate mistake: When Wendy and John are play duelling in their bedroom, in the first shot, John holds the sword in his left hand, but in the next shot, the sword is in his right hand when he hits the bookcase. The second shot is flipped, as we can see by the furniture arrangement and the medal on John's jacket that also switched sides. (00:01:50)
Deliberate mistake: After Peter says, "We can't both have her lady," he shuts the window from the outside and the bottom pane is placed on the outside of the top stained glass pane. This is obviously done specifically for this shot so Peter would be able to shut the window and try to keep the Darlings from opening it. However, in the next shot as the Darlings succeed in opening the 'stuck' window, the bottom pane is on the inside of the top stained glass pane. (00:02:05 - 00:58:45)
Deliberate mistake: After Aunt Millicent checks on the sleeping children, in the shot outside the bedroom, she is startled by her true shadow on the lit area of wall. The position, shape and angle of that area of light, in which the shadow is visible on the wall, is a deliberate exaggeration, not withstanding the fact that Peter, just moments earlier, approached the drawer with his shadow in this same area and yet there was no patch of light. (00:13:15 - 00:14:30)
Deliberate mistake: John yells, "I've got it! I've got it!" jumps onto his bed and we see the length of John's one blanket, mattress and bed frame near the other furniture. In the next two shots as John runs across his bed, the blanket at the foot of the bed is not only pulled all the way out, using what hung over the foot of the mattress in the previous shot, but another blanket is added for extra length. Most blatantly apparent is the span of this mattress and bed frame, to increase the running distance, in relation to the furniture around it. (00:21:35)
Deliberate mistake: After Hook orders the crew into the rigging, Peter lands on deck holding Wendy. There is a close-up of Hook's face as he walks back down the stairs, then the next wide shot of Cookson, the very tall pirate up in the rigging, is flipped. (The shot before he points the gun.) (01:22:25)
Deliberate mistake: The shot just after Smee jumps overboard, is a close-up of Peter's hand twirling his sword in mid-air and grasping the hilt with his left hand. It's not really his left hand though, this shot is flipped. (01:24:50)
Deliberate mistake: Hook tells Peter, "She'd rather grow up than stay with you," and we see that this next wide shot is flipped, note the hook on the left hand. Also, as Hook smacks Peter across his face and sends Peter hurling into the rigging with a bleeding forehead, dramatic as that is Hook doesn't even make contact with Peter in the shot. (01:26:25)
Deliberate mistake: As the Darlings and the Lost Boys laugh in the nursery, the narrator says, "There could not have been a lovelier sight...except a strange boy who was staring in at the window." When the camera pans out through the window Peter's back is visible at the right of the window. This is not Jeremy Sumpter. (01:40:00)
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