Not so long ago, Lillian Kahan would’ve been an oddity.

She’s 104 years old (“104 and a half,” she corrects), and, until recently, a life of such length was a statistical quirk, rare enough to warrant news coverage or scientific research or at least a cupcake at the local senior center.

These days that’s only half true. Kahan’s age still makes news, and scientists increasingly are interested in people like her. She still gets the odd cupcake.

But the attention isn’t coming because she’s so uncommon. It’s because she’s not.

In fact, being a Kahan – living to 100 and beyond – might be a glimpse of the future.

“Being this old is fun,” Kahan said. “I recommend it.”

Welcome to Ageville

The number of centenarians worldwide has more than doubled over the past 25 years and demographers at the United Nations project that the 100-something crowd will quadruple by mid-century. Today, the biggest centenarian populations are in Japan (146,000) and the United States (108,000). But, soon, countries like China and India, where the overall populations are huge but the aging curve is only now starting to trend upward, will have even bigger 100-something age bubbles. By 2054, nearly 4 million people around the world will be 100 or older.

The trend is expected to be even more pronounced locally. The state projects that from now until 2050, the ranks of centenarians will jump more than fivefold in each of Los Angeles, Orange, Riverside and San Bernardino counties.

Of course, centenarians are just the tip of a bigger demographic spear.

Populations are aging up in most advanced economies, at a rate never before seen in human history. In many countries, older people already outnumber children or they’re expected to in the near future. Aging demographics are reshaping everything from retirement plans and immigration patterns to diaper sales and popular ideals about beauty.

Like many aspects of the aging boom, the rise of centenarians is a mixed bag.

For example, it’s unambiguously good that lifestyle changes and cancer prevention and medical sciences have all improved enough to make it possible for so many people to live so long and, often, so well.

“Every centenarian I’ve met is exceptional,” said Stacy Andersen, who, as co-director of the New England Centenarian Study at Boston University, has met a lot of centenarians.

“These people have delayed chronic, age-related diseases. That’s the baseline. But many also continue to live vibrant lives, to stay engaged in their community and with their families,” she added. “It’s a wonderful view of what aging can be.”

It’s also unambiguously great that younger relatives and friends – everybody under 100, really – can, if they listen, pick up some life hacks that come with living 100 or more years.

“I still love waking up,” said Kahan, her New York accent still thick after six-plus decades in Mission Viejo and other parts of Southern California.

“You should try to do that. It’s pretty important.”

But good things often come with a cost, and the rise of centenarians presents some huge challenges.

Money, health, family hassles; the same issues that plague younger people don’t go away when someone turns 100. But the ability to leap over those hurdles – or, perhaps, to worry about them at all – drops considerably when you become a centenarian.

“I don’t necessarily want to make it to 100,” said Margo Carle, an ombudsman with the Council on Aging Southern California who works as an independent advocate for older people who live in nursing homes and other facilities.

“I see too much of how it can be,” Carle said.

“If you don’t have money, being 100 can be … Well, it’s not always pretty.”

Stresses for all

For Kahan and her 100-something cohorts, the cost of living isn’t cheap.

Though studies show centenarians generally are more physically robust than other older people, age is still age. About half of the 100-something crowd in the United States has some form of dementia, and most of those people need full-time care.

And even among those with little or no cognitive decline, only a small fraction can live on their own without someone – paid or otherwise – checking in every day to help them.

“My sons are all teachers. And they’ve arranged their schedules, they rotate, so they can stay with me,” said Rudolph Marcus, a 101-year-old chemist and former Caltech professor who won the Nobel Prize for Chemistry in 1992.

“That helps me continue with my work,” Marcus said. “I still think about my work every day, to be honest. Some problems I can still solve, some I can’t.

“But I could not do any of that without their help.”

Marcus, who won the Nobel for his work on how electrons jump from atom to atom (something that affects the functionality of solar panels and electric cars, among other things), is an outlier. He lives in the same Pasadena house he shared with his late wife, Laura, who died in 2003. Most people his age live in some kind of congregate setting, which in Southern California can run $5,000 to $15,000 a month.

Given that many newly minted centenarians have outlived their retirement savings, or didn’t have much to begin with, the cost of that care often falls to families and the government.

Soon, half of that equation might change.

A proposal being debated in Congress this month could include big cuts to Medicaid, the federal program that helps pay the costs of long-term care for, among others, centenarians. Those cuts, if passed, could result in lower-quality care or, in some cases, displacement. Other proposed Medicaid adjustments could reduce compensation for in-home care, making it tough for centenarians to live without family help.

For families who don’t want to hire out, or who can’t, the costs of centenarian care can run deeper than money.

Unlike younger retirees, whose adult children typically are young enough to still be working, the children of centenarians often are aged themselves – typically in their 70s or 80s. For them, providing care for an aging parent can be devastating, financially and otherwise.

“In many of the cultures that are common in Los Angeles, it’s an honor to care for your aging relatives,” said Heather Cooper Ortner, chief executive of Alzheimer’s Los Angeles, a nonprofit that helps provide services to people and families battling dementia.

“So it isn’t about people being unwilling to do this, or seeing it as a burden. That’s not always the case,” she said.

“But caring for older people can present an incredible level of stress for family caregivers,” Cooper Ortner added. Food, medical questions and appointments, bathing, bathroom help, fall prevention – caring for a centenarian is, literally, a full-time job. For a child in her 80s, it can be too much.

“It’s not uncommon to see a caregiver pass away before the person they’re caring for,” Cooper Ortner said.

“It’s a very complicated dynamic.”

Survive, delay, escape

The first public service messages warning Americans that smoking causes cancer started airing on television in 1967. A few years later, jogging became a national craze and, over the next two decades, about 25 million Americans started going for a run as a regular part of their lives. Less red meat. More sunblock. Meditation. All of it means one thing:

If you’re on deck to turn 100 this year, you’ve spent about half your life in a world where the phrase “healthy lifestyle” wasn’t a punchline.

It’s one reason, though not the biggest, that explains why so many people are living so long. People who study centenarians – and there are hundreds of aging experts looking into the topic in the United States, Japan and Europe – say genetics and the sheer power of population numbers are even bigger factors.

“At the turn of the last century, life expectancy was about 50. But a lot of things – cleaner water, prevention of infant deaths, antibiotics – made it so a lot more people made it into adulthood. That just means there are a lot more people who are going to have the opportunity to hit 100,” said Andersen, of the New England Centenarian Study.

But at least one projection suggests population numbers alone are only part of the broader trend. Even as more people, overall, hit 100, the ratio of people who reach that age is skyrocketing. According to United Nations data, Japan currently has about 12 centenarians for every 10,000 residents (the ratio in the U.S. is about 3 in 10,000). By 2050, the ratio in Japan will be about 40 out of every 10,000, and in the U.S., it’ll be about 14 out of 10,000.

“Having good, healthy habits can get you about 10 years longer. And it definitely makes those years better, which is important,” Andersen said. “But it doesn’t necessarily get you to 100.”

Genes might.

Andersen said there is no single “centenarian gene.” Instead, researchers have identified about 200 different genes to date that do age-related things like reduce inflammation and boost immune systems. People who have certain combinations of those genes have significantly better odds of making it to 100.

“We’re still trying to understand the relationships between protective genes,” Andersen said. “But it’s more about genetics than we once believed. And we’re learning more about that all the time.”

The New England Centenarian Study, which started in 1994, has tracked the lives of more than 1,800 centenarians, including 123 so-called “supercentenarians,” meaning people who made it to 110 or older. It’s also looked at more than 600 of their children, and more than 400 so-called “controlled” subjects, (usually spouses and relatives of spouses), as a way to identify the balance between genetics, lifestyle and other factors when it comes to cracking 100.

They’ve learned, so far, that so-called “exceptional longevity” – meaning the likelihood of making it to 100 — runs in families. They’ve also learned that many people who tend to live so long hit age-related illnesses later in life, and that they often compress their debilitations into shorter windows.

“Centenarians spend about 10% of their lives with a chronic illness. Others spend about 20% of their lives in that kind of situation, on average,” Andersen said.

The study has identified three basic types of centenarians. About 4 in 10 (43%) are “delayers,” meaning they didn’t experience age-related diseases, like dementia, until age 80 or later. Another 4 in 10 (42%) are “survivors,” meaning they made it to 100 even though they’ve been battling some kind of disease since before their 80th birthday. And about 1 in 7 (15%) are “escapers,” or people who, even at 100, don’t have any age-related disease.

Marcus, the chemist from Caltech, is probably an escaper.

“I don’t play tennis anymore. And I don’t ski. My sight doesn’t really allow it. But otherwise I feel pretty much the same,” he said.

When asked if he’s still learning about himself, at age 101, or if he’s got any advice to someone hoping to live well at his age, Marcus said yes and demurred.

“I’m learning every day. I try to live in the moment. I’d like to think I don’t live in the past and I never thought too much about the future, even when I was younger. And I definitely don’t do it now, at my age,” Marcus said, laughing.

“But I wouldn’t know if that’s what other people should or shouldn’t do,” he added. “It’s just the way I’ve always been.”

Kahan is probably a delayer. She doesn’t have dementia, but she said she battles health issues she declined to offer in detail.

She did offer one tip.

“Every day. I watch some TV, I talk with my friend. I enjoy my day,” Kahan said.

“But time passes very quickly,” she added. “Even at my age, it doesn’t slow down. And I think that means something.”

Anyone interested in participating in the New England Centenarian Study can call 888-333-6327 or email agewell@bu.edu.