Juvenile, childish (8)
I believe the answer is:
immature
This clue is a double definition.
'juvenile' is the first definition.
(similar in meaning)
'childish' is the second definition.
(similar in meaning)
(Other definitions for immature that I've seen before include "Lacking in development" , "Undeveloped - unripe" , "Underdeveloped - childish" , "Not yet fully-developed" , "Not yet adult or fully grown" .)