The reference is: Circular AIC A 21/00 AUG17 issued by the Direction Generale de l'Aviation Civile.
The English translation states: ' . . . in conformity with ICAO Annex 1 . . . acting as a commander on international air transport flights in French airspace is only authorised to those captains who are under the age of 60.'
Interestingly it specifically refers in the English and French versions to 'international' flights so one assumes that a French captain over 60 could command an internal public transport flight.