Typically too old to absorb sodium (2,7)
I believe the answer is:
on average
'typically' is the definition.
(I've seen this in another clue)
'too old to absorb sodium' is the wordplay.
'too old' becomes 'overage' (I've seen this before).
'to absorb' indicates putting letters inside.
'sodium' becomes 'NA' (chemical symbol).
'overage' going around 'na' is 'ON AVERAGE'.
(Other definitions for on average that I've seen before include "Generally" , "Usually" , "typically speaking" .)