"I think when you consider the sincerity of the man, when you consider the weight of the universal church, and the greatest variety of issues affecting the church and the rest of the world, I could see how he could come to that decision," Elizondo told ABCNews.com."What's really needed is a younger person with more vigor and up-to-date knowledge about what's happening. I think that's the rationality behind this pope."
Pope Benedict XVI was just 73 days away from being the third oldest pope, but he will remain the fourth oldest pope because his resignation date is scheduled for Feb. 28, said Guruge. The three older popes were Pope Clement X, who lived to be just over 86 years old; Pope Clement XII, who lived to be 87; and Pope Leo XIII, who lived to be 93. They all died without resigning.
Pope Leo III, who died in 1903, became known as the "eternal pope" because he kept on living, Guruge said. Then, however, the job was less demanding because the pope didn't have to be on television or travel -- or tweet.
Although church law says that cardinals should submit their resignations to the pope when they reach 75 years old, there is no such law for the pope, said Father Francis Schussler Fiorenza, who teaches Roman Catholic theological studies at Harvard University. Fiorenza studied under Pope Benedict XVI about 40 years ago in Munster, Germany, when the pope was still Father Joseph Ratzinger.
"I know what he looked like when he was really young and healthy," Fiorenza told ABCNews.com, adding that Pope Benedict XVI's predecessor, Pope John Paul II, was sick until he died in 2005 at 85 years old.
Pope John Paul II lived with Parkinson 's disease for decades, but he died of cardio-circular failure and septic shock on April 2, 2005. He also had kidney failure.
New York Cardinal Timothy Dolan told ABC News that he trusts Pope Benedict XVI's judgment on stepping down, but he was impressed with the 85-year-old's ability to stay awake during a church leaders' meeting.
"I was amazed. He was there every day. Never fell asleep," Dolan said with a laugh. "I can't say that about myself in some of the talks."