By Philip Pullella
VATICAN CITY (Reuters) - Pope Benedict marked his 85th birthday on Monday saying he is now in the "last stretch" of his life but sure that God would help him continue his mission.
Benedict, who has looked tired and drawn recently, is one of history's oldest reigning pontiffs - and already older than his predecessor John Paul II was when he died in 2005.
The pope said a Mass of thanksgiving with German bishops and close aides. Afterwards, children in traditional garb from his native Bavaria danced for him under the frescoes of the Vatican's Clementine Hall.
"I find myself facing the last stretch on the road of my life," Benedict said in German during the early morning Mass in a Vatican chapel.
He said he was confident that God's light would help him "proceed with assurance".
Benedict is now the oldest reigning pope since Leo XIII, who died aged 93 in 1903 after reigning for 25 years.
On Thursday, Benedict will mark another milestone when he celebrates the seventh anniversary of his election.
The pope has looked particularly tired since his return from a grueling trip to Mexico and Cuba last month, followed by a hectic week of ceremonies leading up to Easter. But he was alert and smiling on Monday.
Speaking to pilgrims and tourists in St Peter's Square on Sunday, he asked for prayers "so that the Lord may give me the strength to carry out the mission he has entrusted to me".
Benedict's older brother, Monsignor Georg Ratzinger, who has often expressed concern for the health of his younger sibling, flew in for the celebrations, which were also attended by some Bavarian politicians.
"I think he won't travel that much anymore, because it's more and more of an effort," Georg Ratzinger, 88, told the German Catholic news agency KNA earlier this month.
The Vatican has announced that Benedict will visit Lebanon, only his second trip this year, while he has made more trips per year in past years.
When he left for Mexico and Cuba, he used a cane at the airport for the first time in public, though sources say he has been using it in private for some time.
Last year, to conserve his strength, he began using a mobile platform instead of walking up the aisle of St Peter's Basilica.
The Vatican says it is to spare him fatigue and that there is no concern about his overall health. His brother has said Benedict suffered two mild strokes before his election in 2005 and he reportedly suffers from high blood pressure and arthritis.
The two milestones this week have again prompted speculation that Benedict might one day become the first pontiff in seven centuries to resign willingly.
One retired archbishop, Luigi Bettazzi of the northern city of Ivrea, openly speculated that someday Benedict could choose to step down instead of reigning for life if he felt he could no longer run the Church properly.
"Those of us who are over 75 are not allowed to run even a small diocese and cardinals over 80 are not allowed to elect a pope. I can understand why one day the pope might say 'even I can't do my job anymore,'" he said during a program about the pope on Italian state RAI television on Saturday night.
"I wish him a long life and lasting lucidity but I think that if the moment arrives when he sees that things are changing, I think he has the courage to resign," said Bettazzi, who has known the pope for 50 years.
Benedict is the only pope in living memory to discuss publicly the possibility of resignation, though others have done so privately.
In a book in 2010, he said he would not hesitate to resign if he felt no longer able "physically, psychologically and spiritually" to run the Catholic Church.
The last pope to resign willingly was Celestine V in 1294 after reigning for only five months. Gregory XII reluctantly abdicated in 1415 to end a dispute with a rival claimant to the papacy.
(By Alessandra Rizzo)