Pope Francis, who turns 88 on Dec. 17, is the second-oldest pope in modern history (after Pope Leo XIII, who died in 1903 at the age of 93). Pope Benedict XVI resigned at the age of 85 and went on to live almost another decade as pope emeritus. Pope St. John Paul II was almost 85 when he died in 2005.

The world is getting used to older leaders, both inside and outside the Church. President-elect Donald Trump will be almost 83 — and the oldest president in U.S. history — at the end of his term in 2029. It’s increasingly normal to see octogenarians and even nonagenarians continuing to work in various fields.

Yet longer lifespans raise the prospect of increased debility. President Joe Biden withdrew from his reelection campaign in July after evidence of age-related decline made victory seem impossible. St. John Paul struggled with Parkinson’s disease for the last years of his life. And when Pope Benedict stepped down in 2013 — the first pontiff to do so in nearly six centuries — he cited “the increasing burden of age,” which he said had left him too weak to carry out the duties of a 21st-century pope.

Pope Benedict suggested in his resignation speech that the mental and physical demands of the papacy had increased “in today’s world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith.” He later said that he retired when he did after his doctor warned him against flying to Brazil for World Youth Day — not a task in the original job description for successors of St. Peter.

Grappling with the question of aging leadership has been part of the Church’s efforts to adjust to the modern world. The Second Vatican Council (1962-65) introduced the concept of retired bishops, calling on the leaders of dioceses to resign if they became “less capable of fulfilling their duties properly because of the increasing burden of age or some other serious reason.” Until then, lifelong tenure for bishops had been the norm. St. Paul VI established the practice of bishops turning in their resignations at the age of 75.

St. Paul VI also made cardinals ineligible to vote in a conclave for pope after they turned 80. That decision was controversial at the time. “It is an act committed in contempt of tradition that is centuries old,” said Cardinal Alfredo Ottaviani, who had been a leading conservative at Vatican II. “Over the centuries, in fact, it has been immutably held that, indeed, advanced age guaranteed to the Church counselors rich in experience, certainty, prudence, and doctrine.”

But the age limit on voting cardinals is useful to popes, allowing them to remake the electorate according to their personal vision.

Pope Francis has now named 79% of the body that would elect his successor were a conclave to be held today. In the process, the first pope from the Global South has reduced the proportion of cardinal electors who hail from the Church’s historic heartland of Europe to 39%, from 52% at the 2013 conclave, and has tended to favor theological and political progressives over conservatives in his nominations.

Pope Francis has been less sanguine about a mandatory retirement age for popes.

“One could imagine it, but the idea of setting an age limit does not appeal to me, because I believe that the papacy has an element of being the final authority,” he told an interviewer in 2015. According to canon law, the pope “possesses supreme, full, immediate, and universal ordinary power in the Church, which he is always able to exercise freely.”

So any retirement age he might set for himself would be no more than a suggestion to future pontiffs.

Pope Francis has repeatedly praised his predecessor for opening the door to papal resignation and has said he would step down himself if he saw the need. He has said that he signed a letter of resignation to take effect if he should become incapacitated, a measure also taken by Sts. Paul VI and John Paul II, though it is not clear what legal value such a document would have.

Pope Francis, who has twice undergone abdominal surgery during his papacy, has been hospitalized for respiratory problems, and has used a wheelchair since May 2022, is more obviously impaired than Pope Benedict was when he stepped down. Yet earlier this year Pope Francis dismissed speculation that he was considering resignation, and he has been highly active since.

In September he took the longest trip of his pontificate — an 11-day marathon through Asia and Oceania — and in October he oversaw a month-long synod at the Vatican. He is scheduled to visit the French island of Corsica two days before his birthday this month and recently said he plans to travel to Turkey to mark the 1,700th anniversary of the Council of Nicaea in 2025.

“In order to govern the barque of St. Peter and proclaim the Gospel, both strength of mind and body are necessary,” Pope Benedict said when he announced that he would resign. His successor seems determined to show that he still has both.