Four keys, getting old (4)
I believe the answer is:
aged
This could be a double definition.
'four keys' is the first definition.
'aged' can be an answer for 'keys' (A, G, E and D are musical keys). I am not sure about the 'four' bit.
'old' is the second definition.
(similar in meaning)
'getting' is the link.
Can you help me to learn more?
(Other definitions for aged that I've seen before include "Long in the tooth" , "Old or ancient" , "Grew older" , "way over the hill" , "not a modern copy" .)