Can a 79 year old rent a car in Nice to drive to Paris?
Renting a car at 79 is not a problem in France.
But, see your other thread: driving from Nice to Paris is long and boring unless you are making a road trip out of it and planning several stops in areas away from the main Nice - Aix/Marseille - Avignon - Lyon - Dijon - Paris axis.
We found that it depends on the car rental company you book with. Some restrict age of drivers, others don’t.
We drove Paris to Nice but took three weeks stopping in Burgundy and Provence en route.