Old and mean, but not initially too old (7)
I believe the answer is:
overage
'too old' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for overage that I've seen before include "Too old to qualify or a surplus of goods" , "Excess" , "Older than certain limit" , "Excessively old" , "Too old to be eligible" .)