Eternally youthful (7)
I believe the answer is:
ageless
'eternally youthful' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ageless that I've seen before include "not appearing to get old" , "not getting on" , "Eternally young" , "unaffected by time" , "Enduring" .)