The average age in the interim (8)
I believe the answer is:
meantime
'the interim' is the definition.
(meantime is a kind of interim)
'the average age' is the wordplay.
'the average' becomes 'mean' (mean is a kind of average).
'age' becomes 'time' (I've seen this before).
'mean'+'time'='MEANTIME'
'in' acts as a link.
(Other definitions for meantime that I've seen before include "Intervening period" , "interim?" , "For the moment" , "interval" .)