Growing old (6)
'growing old' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aging that I've seen before include "development" , "Getting old" , "Maturing" , "in decline" , "Getting older" .)
'growing old' is the definition.
('age' can be a synonym of 'grow old')
This is the entire clue.
(Other definitions for ageing that I've seen before include "getting more advanced?" , "process of development" , "Getting older" , "Declining" , "rather old" .)