How old? About 49 — but nimble (5)
I believe the answer is:
agile
'nimble' is the definition.
(agile can mean nimble or athletic)
'how old? about 49' is the wordplay.
'how old?' becomes 'age' (I've seen this before).
'about' means one lot of letters goes inside another.
'49' becomes 'IL' (Roman numerals).
'age' placed around 'il' is 'AGILE'.
'but' is the link.
(Other definitions for agile that I've seen before include "Dexterous" , "Quick-moving, supple" , "Physically quick and nimble" , "alert" , "Nimble on the feet" .)