I would think that any pope who resigned simply because of reaching a certain age would be setting a precedent which future popes would have a hard time ignoring. Resigning because of incapacity to carry out the duties of the office would be a different matter.
Good point. However, in contemporary times, we find people live longer and the pope, thanks to modern transportation, has a much more strenuous workload than the majority of his predecessors who rarely traveled beyond Italy.
There is a recent precedent, though on a different level. The Patriarch of the Maronite Catholic Church, Nasrallah Cardinal Sfeir, submitted his resignation in January of this year at the age of 90. He and Pope Benedict were good friends and, like Ratzinger, Sfeir was in good health with all of his faculties. When Pope Benedict traveled to Australia, Sfeir accompanied him there to meet with his Maronite community. I was amazed to see Sfeir at age 87, bound down the ship's access way and greet the crowd waiting to welcome him.
In his letter of acceptance, Pope Benedict noted Cardinal Sfeir's service to the Catholic Church.
You could celebrate last year's sixty years of your priesthood: proof of loyalty and love for Jesus Christ, the High Priest. In July, you will again have the opportunity to raise a thanksgiving to the Holy Trinity for the completion of fifty years as bishop.For nearly twenty-five years, you worked with your two predecessors in the See of Antioch before being chosen by the Synod to succeed them April 19, 1986: a turning point that puts you on the threshold of your Silver Jubilee in this office. Full Text.
A synod was convened and bishops from around the world arrived in Bkerke, Lebanon to elect a successor to Sfeir. They chose Bechara Boutros al-Rahi. At 71, he was elected Patriarch of the Maronites on 15 March 2011, after getting more than two-thirds of the votes of the 39 bishops and replacing Nasrallah Sfeir. The Mass for the inauguration of his patriarchate took place on 25 March 2011, in Bkerké, the see of the Maronite Catholic Patriarchate. As is customary for all Maronite patriarchs Rahi took the additional name Boutros, that of Saint Peter, who briefly held the See of Antioch before moving to Rome to become bishop there.