LONDON -- When he became pope at age 78, Benedict XVI was already the oldest pontiff elected in nearly 300 years. He's now 85, and in recent years he has slowed down significantly, cutting back his foreign travel and limiting his audiences.

The pope travels to the altar in St. Peter's Basilica on a moving platform to spare him the 100-yard (-meter) walk down the aisle. Occasionally he uses a cane. Late last year, people who were spending time with the pontiff emerged saying they found him weak and too tired to engage with what they were saying.

The Vatican stressed on Monday that no specific medical condition prompted Benedict's decision to become the first pontiff to resign in 600 years. Still, Benedict said his advanced age means he no longer has the necessary mental and physical strength to lead the world's more than one billion Roman Catholics.

That Benedict is tired would be a perfectly normal diagnosis for an 85-year-old pope, even someone with no known serious health problems and a still-agile mind.

He has acknowledged having suffered a hemorrhagic stroke in 1991 that temporarily affected his vision, but he later made a full recovery. In 2009, the pope fell and suffered minor injuries when he broke one of his wrists while vacationing in the Alps.

A doctor familiar with the pope's medical team told The Associated Press on Monday that the pontiff has no grave or life-threatening illnesses. But, the doctor said, the pope - like many men his age - has suffered some prostate problems. Beyond that, the pope is simply old and tired, the doctor said on condition of anonymity.

According to the pope's brother Georg Ratzinger, the pontiff was told by his doctor not to take any more trans-Atlantic trips. In fact, the pontiff's only foreign trip this year was scheduled to be a July visit to Brazil for the church's World Youth Day.

Experts weren't surprised the pope's health problems were slowing him down.

"In someone who's 85 and has arthritis, the activities of being a pope will be a struggle," said Dr. Alan Silman, the medical director of Arthritis Research U.K. He said Pope Benedict most likely has osteoarthritis, which causes people to lose the cartilage at the end of their joints, making it difficult to move around without pain.

"It would be painful for him to kneel while he's praying and could be excruciating when he tries to get up again," Silman said, adding that for people with arthritis, even standing for long periods of time can be challenging.

Silman said some drugs could help ease the pain, but most would come with side effects such as drowsiness or stomach problems, which would likely be more serious in the elderly.

The doctor said it isn't clear whether the pope's arthritis would worsen with age. "It could be it's as bad as it's going to get," he said. "But it already sounds like he has it pretty bad and continuing with all the activities of being the pope won't help."

Joe Korner, a spokesman for Britain's Stroke Association, said having a mild stroke also could be a warning of a possible major stroke in the future. "I would imagine the pope has been warned this could happen and that he should make some changes to his lifestyle," Korner said, including reducing stress levels.

When he became pope, Benedict replaced John Paul, who died in 2005 at the age of 84. He was the Vatican's most-traveled pontiff, visiting 129 countries during his nearly 27-year papacy and had captured the world's affection like no other pope.

In the last year of his life, John Paul was forced to curtail his travels because of old age and illness, including trembling hands and slurred speech, an inability to walk or hold his head up, and other symptoms of Parkinson's disease.

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Nicole Winfield in Rome contributed to this story.