Lou Weintraub, who turned 100 in January, works out recently at a gym in Sacramento, Calif. Though his eyesight is failing, Weintraub is still leads an active life, like many Americans who are older than 100.

SACRAMENTO, Calif. — After celebrating his 100th birthday in Sacramento, Calif., with friends and family, Lou Weintraub headed to sea for a 10-day cruise with his wife.

“You should have seen the number of older people on the cruise,” said Weintraub, who retired from work as a nonprofit executive in 1979.

“They weren't older than you,” said his wife, Roz Levy-Weintraub, 82, who still works selling real estate.

“But they looked older,” said Weintraub.

“And they seemed older,” agreed Levy-Weintraub.

He was born to Polish immigrant parents in New York on Jan. 25, 1914, the middle of their three offspring. Weintraub was a child of the hard Depression years, and later, as a young man, he served as a clinical psychologist in the military during World War II. Sharp and involved in the community as a volunteer even today, he remembers it all.

As a centenarian, Weintraub is part of another significant moment: the nation's rapid demographic shift into very old age. According to the 2010 U.S. census, America is home to the world's largest population of centenarians, more than 53,300 people 100 years old and older.

That number represents an astonishing 66 percent increase over the nation's centenarians in 1980. During the same time, the country's total population grew by 36 percent.

And California leads the way with the oldest of the old: In 2010, according to the census, the state was home to 5,921 centenarians, or more than one-tenth of the nation's total population 100 years old and older.

Celebrating a 100th birthday — that special milestone for centenarians and their loved ones — is becoming statistically commonplace.

“We've seen life expectancy make considerable gains in the past 100 years,” said Joe Rodrigues, long-term care ombudsman for the California Department of Aging. “We used to say the fastest-growing segment of our oldest adults was 85 and older. Today, it's 100 and older.”

People are paying attention to that statistical reality: To recognize people turning 100, Assembly member Mariko Yamada of Davis, Calif., established what she calls the Century Circle of centenarians in her district. Since 2010, her office said, she has honored 91 100-year-olds. And there are enough people living to 100 that the Social Security Administration has created the Centenarian Project, which seeks to verify that centenarians receiving benefits are really still alive.

Experts on aging know that centenarians are most likely to be female and white, residents of the West or the South, and living on their own or with family members, not in nursing care. But why are so many more people today living so much longer?

Winning the genetic lottery plays a big role — Weintraub's older sister, for example, lived to age 102. But genes aren't the whole story.

“Genes are 30 percent of healthy aging,” said Cheryl Osborne, chairman of the California State University-Sacramento gerontology program. “The other 70 percent involves what we do with what we've got.

“To live to 100, you have to be well physically and socially and psychologically and spiritually. These are not people who are dying in skilled nursing. They're actively living in the community.”

And they've been lucky. To live to 100, today's centenarians had to survive their earliest years: More than 25 percent of children born in the early 1900s died before they reached school age. Life at the turn of the last century was hard, and often, it was short. The average life expectancy was 47.

Today's centenarians had to survive the war years, as well as the diseases that in the 1960s struck down so many of their generation in middle age, such as heart attack and cancer.