© Honda/AFP/GettyPope Benedict XVI waves as he leaves Yankee Stadium after saying mass on April 20, 2008 in New York.
A decline has been noted as Benedict prepares for next weekend's grueling Christmas celebrations, which kick off two weeks of intense public appearances.

Pope Benedict XVI seems worn out.

People who have spent time with him recently say they found him weaker than they'd ever seen him, seemingly too tired to engage with what they were saying. He no longer meets individually with visiting bishops. A few weeks ago he started using a moving platform to spare him the long walk down St. Peter's Basilica.

Benedict turns 85 in the new year, so a slowdown is only natural. Expected. And given his age and continued rigorous work schedule, it's remarkable he does as much as he does and is in such good health overall: Just this past week he confirmed he would travel to Mexico and Cuba next spring.

But a decline has been noted as Benedict prepares for next weekend's grueling Christmas celebrations, which kick off two weeks of intense public appearances. And that raises questions about the future of the papacy given that Benedict himself has said popes should resign if they can't do the job.

Vatican spokesman the Rev. Federico Lombardi has said no medical condition prompted the decision to use the moving platform in St. Peter's, and that it's merely designed to spare the pontiff the fatigue of the 100-meter (-yard) walk to and from the main altar.

And Benedict rallied during his three-day trip to Benin in west Africa last month, braving temperatures of 32 Celsius (90F) and high humidity to deliver a strong message about the future of the Catholic Church in Africa.

Wiping sweat from his brow, he kissed babies who were handed up to him, delivered a tough speech on the need for Africa's political leaders to clean up their act, and visited one of the continent's most important seminaries.

Back at home, however, it seems the daily grind of being pope - the audiences with visiting heads of state, the weekly public catechism lessons, the sessions with visiting bishops - has taken its toll. A spark is gone. He doesn't elaborate off-the-cuff much anymore, and some days he just seems wiped out.

Take for example his recent visit to Assisi, where he traveled by train with dozens of religious leaders from around the world for a daylong peace pilgrimage. For anyone participating it was a tough, long day; for the aging pope it was even more so.

"Indeed I was struck by what appeared to me as the decline in Benedict's strength and health over the last half year," said Rabbi David Rosen, who had a place of honor next to the pope at the Assisi event as head of interfaith relations at the American Jewish Committee.

"He looks thinner and weaker ... which made the effort he put into the Assisi shindig with the extraordinary degree of personal attention to the attendees (especially the next day in Rome) all the more remarkable," Rosen said in an email.

That Benedict is tired would be a perfectly normal diagnosis for an 84-year-old, even someone with no known health ailments and a still-agile mind. He has acknowledged having suffered a hemorrhagic stroke in 1991 that temporarily affected his vision. And his older brother, who has a pacemaker for an irregular heartbeat, has expressed concern about Benedict's own heart.

But Benedict is not a normal 84-year-old, both in what he is called to do and the implications if he were to stop.

Popes are allowed to resign; church law specifies only that the resignation be "freely made and properly manifested."

Only a handful have done so, however. The last one was Pope Gregory XII, who stepped down in 1415 in a deal to end the Great Western Schism among competing papal claimants.

There's good reason why others haven't followed suit: Might the existence of two popes - even when one has stepped down - lead to divisions and instability in the church? Might a new resignation precedent lead to pressures on future popes to quit at the slightest hint of infirmity?

Yet Benedict himself raised the possibility of resigning if he were simply too old or sick to continue on, when he was interviewed for the book Light of the World, which was released in November 2010.

"If a pope clearly realizes that he is no longer physically, psychologically and spiritually capable of handling the duties of his office, then he has a right, and under some circumstances, also an obligation to resign," Benedict said.

The former Cardinal Joseph Ratzinger had an intimate view as Pope John Paul II, with whom he had worked closely for nearly a quarter-century, suffered through the debilitating end of his papacy. After John Paul's death at age 84, it was revealed that he had written a letter of resignation to be invoked if he became terminally ill or incapable of continuing on.

And it should be recalled that at the time Benedict was elected pope at age 78 - already the oldest pope elected in nearly 300 years - he had been planning to retire as the Vatican's chief orthodoxy watchdog to spend his final years writing in the "peace and quiet" of his native Bavaria.

It is there that his elder brother, Monsignor Georg Ratzinger, still lives. Ratzinger, who turns 88 next month, is nearly blind. Benedict has said his brother has helped him accept old age with courage.

Benedict said in Light of the World that he knew his own strength was diminishing - steps are difficult for him and his aides regularly hold his elbows as he climbs up or down. But at the same time Benedict insisted that he had no intention of resigning to avoid dealing with the problems of the church, such as the sex abuse scandal.

"One can resign at a peaceful moment or when one simply cannot go on. But one must not run away from danger and say that someone else should do it," he said.

As a result, a papal resignation anytime soon seems unlikely.

And Benedict is maintaining a hectic agenda. His planned trip to Cuba and Mexico next spring will fall shortly before he turns 85 on April 16. He has also said he'd like to make it to Rio de Janeiro in 2013 for the next World Youth Day.

Sometime in the New Year he will presumably preside over a new consistory to name the new cardinals who will elect his successor. And he has lots of unfinished business close to his heart: Bringing back breakaway traditionalists under Rome's wing, the fate of the sex abuse-scarred Irish church, tensions with China.

And he still cuts a robust figure in public given his age, walking briskly, speaking clearly and emphasizing key points. But his public engagements have been trimmed back; he had far fewer speeches in Benin than during his September visit to his native Germany or the United Kingdom last fall.

And behind closed doors, during audiences without the glare of TV cameras or throngs of the faithful encouraging him on, he has begun to show his age, acquaintances say.

The Rev. Joseph Fessio, Benedict's U.S. publisher and onetime student, sees the pope every so often, including during the summer when Benedict gathers his former theology students for an informal academic seminar at the papal summer retreat in Castel Gandolfo.

Fessio recalled a day in the 2010 edition that remains with him: "In the Saturday morning session, the pope looked older and weaker than I had ever seen him before. In fact I remarked to someone that it's the first time I've seen him look like the old man that he is. He was speaking in softer tones than even his normally soft speaking voice. His head was bowed. He was pale. He just looked frail."

But then, after lunch and an apparent rest, Benedict returned for the afternoon session. "It was a complete transformation. He was lively, vigorous, attentive, and with his usual good humor," Fessio said.

Clearly, at his age Benedict has good days and bad, even good half-days and bad.

Yet he's never called in sick. In fact as pope, he has only had one significant known medical incident: He broke his right wrist when he tripped on the leg of his bed and fell while on vacation in the Alps in 2009.

Lombardi says the pope realizes the limitations of his strength, and that's why the recent trip to Benin was a one-stop-only affair.

"I think it's an example of the great willingness and wisdom of the Holy Father to continue doing these trips, even those that are difficult or far away," Lombardi said. He said the pope "measures well what his strengths are, and the possibility of doing the trips well."

"When I'm 84 I think I'll have been buried for many years," he added.

But he refused to give any kind of medical updates on the pope.

"I'm not a doctor. I don't give medical bulletins," Lombardi said. He paused, then added quietly: "In this phase. At this moment."