Pope John Paul II was elected at the age of 58, and served 20 years in reasonable health, before declining quickly in his late 70’s. Pope Benedict was only 7 years younger than John Paul II, and retired when he was two years older than John Paul II at his death. Francis is now two years year younger than John Paul II at his death.
Health can decline rapidly and unpredictably after age 75. It would not be inappropriate for the a similarly aged man to retire before his faculties fade. In the distant past, pope’s did not have advanced medicine, so advanced service while incapacitated by illness was relatively rare. I would argue it would be best for a Pope to retire when his faculties are fully intact, so that their is no reasonable doubt as to the free consent to resign. Retiring in good health allows for the orderly transition of church health.