Are over age (3)
I believe the answer is:
era
'age' is the definition.
(both can mean a long time)
'are over' is the wordplay.
'over' says the letters should be written backwards.
'are' back-to-front is 'ERA'.
(Other definitions for era that I've seen before include "Memorable period" , "Important historical period" , "Distinctive period of time" , "day" , "Age of importance" .)