JACKSONVILLE -- Republican presidential hopeful John McCain is asked constantly about his 71 years on earth. First, he responded with jokes. Now, he talks about judgment.

In the early days of his campaign, McCain kidded: "I'm older than dirt, more scars than Frankenstein."

Today, with the race at a critical stage — and with recent polls putting him toward the head of the Republican pack — the Arizona senator puts kidding aside.

"I can out-campaign anybody that's running, because I love it. It invigorates me," McCain told a town hall meeting Saturday in Fort Myers, Florida.

"Seriously, can I say, right now, we need judgment," McCain said. "We need judgment. We live in a very, very dangerous and challenging world. I've been involved in every major national security issue facing this country, including having the pride of raising my hand at age 17 that I would support and defend the constitution of the United States."

These days, instead of joking, McCain is more likely to urge people to meet his 95-year-old mother, Roberta. "Last Christmas, she wanted to drive around France. So she flew to Paris and tried to rent a car. They said she was too old, so she bought one and drove around France."

The questions come up again and again because of McCain's age and his three bouts with melanoma, an aggressive and potentially deadly form of skin cancer. It came up Saturday in a newspaper endorsement from the St. Louis Post-Dispatch.

The newspaper wrote that, despite its approval, "we also have concerns about Mr. McCain's age (he will be 72 at the time of the Republican convention) and his health. ... That would make his choice of a running mate a matter of even greater national concern."

If elected, McCain would be the oldest man to become president of the United States; Ronald Reagan was 69 when he took office in 1981.

In Florida, site of Tuesday's primary election and home of the country's highest concentration of people 65 and older, many didn't care about McCain's age.

"If you can take the haul of a campaign, you can probably manage the White House," said Nancy Swallow, 72, a McCain supporter from Lake Wells, Florida. "I think anybody has got to be off their rocker to run for president, because it's a horrible, grueling thing to do."

McCain is notorious for his vigor and quick wit. From the moment he boards his campaign bus for 12 hours or longer, McCain is in ceaseless campaign mode. He answers questions from voters at freewheeling town hall meetings; in between stops, he usually fields questions and banters with journalists on his bus.

"Nobody does this. He's amazing that way," said Sen. Joseph Lieberman, who wedged himself into the horseshoe-shaped space at the back of McCain's bus on Saturday for an hour and a half. McCain took questions the entire time.

The 2000 Democratic vice presidential nominee and now an independent who has endorsed McCain, Lieberman said he sometimes couldn't help but doze between events.

Aides try to force McCain to take breaks, but aside from an occasional nap on a plane, McCain almost never rests. "Can't they have a slice of pizza?" he implored an aide trying to usher reporters off the bus after his final campaign event Saturday.

McCain stays active outside politics, too; he hiked the Grand Canyon from rim to rim last summer.

McCain bears obvious signs of his age, though his face is less wrinkled and lacks the jowls of some younger men. Age spots are visible on his hands, featured prominently in his portrait on the cover of last week's Time magazine, and the left side of his face bears a scar and swelling from his cancer.

McCain's movements — walking, using his arms — show signs of a battle he fought long before the melanoma. A Navy fighter pilot captured by the Vietnamese when his plane went down over Hanoi in 1967, McCain broke both arms and hurt his right knee. The injuries were never properly cared for and worsened by torture during his more than five-year imprisonment.

As a result, McCain has a slight limp, cannot raise his arms above his head and has arthritis.

Exhaustive health records were made available during McCain's first campaign for president in 2000, and his campaign intends to make updated records available. McCain aides said he has been cancer-free for more than five years, although McCain, like all cancer survivors, faces a higher risk of recurrence.

McCain's doctors consider him to be in excellent health, according to a medical summary his campaign provided last week says; the summary said McCain currently takes the cholesterol drug Vytorin, baby aspirin and a multi-vitamin, and occasionally some prescription allergy medicines.

Despite his tirelessness, McCain seemed weary of yet more questions about his age on Saturday on his campaign bus. He tried to bring the issue around to his judgment.

But how old does he feel?

"I'm continuously surprised that I'm on this earth, and so are people who know my life," McCain said. "I have been the most fortunate person that has ever lived. OK? I am the most fortunate person that you will ever know. I've survived in situations that defy all odds. So I am grateful for every single day, and I know that you can't ever live those days over again.

"And I am grateful for the opportunity to serve. That's why you see my line that I use very often — I'd like to serve a little while longer."