Inexperienced or unsophisticated, as a youth say (6)
I believe the answer is:
callow
(Other definitions for callow that I've seen before include "Inexperienced or innocent" , "Simon ---, actor; immature" , "lacking experience of life" , "Inexperienced or unsophisticated, as a youth say" , "Immature and inexperienced, as youth often is" .)