This 60 year old rule was introduced by the unions when AIR FRANCE took over AIR INTER.
AIR FRANCE pilots had always retired at 60 but AIR INTER pilots not and the big deal was about seniority list.
By making it illegal to work above 60 the AIR FRANCE unions solved the problem the usual way, that is by depriving all the non AF pilots the right to work after 60.
Thank you AF pilots...