Decline of French people of similar age (12)
I believe the answer is:
degeneration
'decline' is the definition.
(I've seen this before)
'of french people of similar age' is the wordplay.
'of french' becomes 'de' ('of' in French).
'people of similar age' becomes 'generation'.
'de'+'generation'='DEGENERATION'
(Other definitions for degeneration that I've seen before include "Degradation" , "The decline" , "Atrophy" , "Decadence" .)