Boy who never grew up (5,3)
I believe the answer is:
peter pan
'boy who never grew up' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for peter pan that I've seen before include "ageless character" , "J. M. Barrie play" , "J M Barrie's immortal boy" , ",one with youthful persona, maybe" , "Type of eternal youth" .)