When Mack Gold turned 100 years old, he celebrated by doing a few push-ups in the middle of the living room. That was three years ago.

Now 103, the recipient of a Purple Heart in World War II is no longer up to an all-out aerobic workout, but he keeps a positive attitude as he rolls toward his goal of becoming the oldest person in the U.S.

"If it works out, fine," said Gold. "If not, I'm still proud of myself."

The competition is fierce. As the nation marks May as Older Americans Month, there are more than 72,000 centenarians in the U.S., according to the Census Bureau. As many as 4,000 are residents of Florida.

The century club is growing by the day. Indeed, Americans over 100 make up the second fastest-growing segment of the population. The only group growing more rapidly: supercentenarians, those at least 110 years old.

The world's oldest person, at 114, is believed to be Bessie Cooper, of Monroe, Ga.

At 102, Ruth Greenstein of Delray Beach said she does not know how long she will be around, but she plans to stay busy designing and making jewelry and playing bingo.

At Heritage Park East, an assisted living facility, Greenstein also loves to talk, especially of the trip she made to the U.S. from her native Poland as an unaccompanied 14-year-old.

"She is a remarkable woman," said Greenstein's daughter, Fran Weinberg, 78, of Boynton Beach. "She loves to be with people, she reads a little and tries to keep up."

Reaching 100 is not the oddity it once was. Hallmark has been printing milestone "Happy 100th Birthday" cards since 1986. A cake with 100 candles no longer guarantees a photo in the local newspaper.

Nor does reaching an advanced age necessarily mean disability or loss of mental acuity. It doesn't always even mean retirement: In Wichita, Kan., U.S. District Judge Wesley Brown remains on the federal bench at 103.

Among newcomers to South Florida's century community is Vincent Giacalone of Hallandale Beach, who marked his 100th birthday on Feb. 25. Like Gold, Giacalone is a veteran of World War II, among the oldest of those drafted into the Army after the U.S. entered the conflict in 1941.

Giacalone — called Jimmy — dances, cooks spaghetti and other dishes from his native Italy and goes to Gulfstream Park every weekend to play the ponies.

"I take care of myself," said Giacalone, a former barber. "What I don't do is sit in front of the television and sleep."

Gerontologists and other researchers have come up with many theories to explain increasing longevity, including improved medical science, social engagement, physical exercise, the ability to handle stress and good luck.

In their book "The Longevity Project," Howard S. Friedman and Leslie R. Martin called on eight decades of research to conclude that "the best childhood personality predictor of longevity was conscientiousness, the qualities of a prudent, persistent, well-organized person, like a scientist-professor — somewhat obsessive and not at all carefree."

Many researchers think genetics are key; if your parents lived long lives, so might you.

"We age at different rates, and that is a consequence of genetic make-up, along with our interaction with the environment," said Nir Barzilai, director of the centenarian study at Albert Einstein College of Medicine of Yeshiva University, in New York City.

"Ask yourself if you know someone in their 60s who looks like they are in their 50s. Most people do, and that is mindblowing to me. It shows that people intuitively understand that we age at different rates."