Not as young (5)
I believe the answer is:
older
'not as young' is the definition.
('old' can be the opposite of 'young' so perhaps 'older' is reasonable for 'not as young')
This is all the clue.
(Other definitions for older that I've seen before include "Earlier" , "Elder" , "More ancient" , "Advanced in years (but wiser?)" , "Of greater age (O, not E)" .)