Washington ? It’s starting to get crowded in the 100-year-olds’ club.

Once virtually nonexistent, the world’s population of centenarians is projected to reach nearly 6 million by midcentury. That’s pushing the median age toward 50 in many developed nations and challenging views of what it means to be old and middle-age.

The number of centenarians already has jumped from an estimated few thousand in 1950 to more than 340,000 worldwide today, with the highest concentrations in the U.S. and Japan, according to the latest Census Bureau figures and a report being released Monday by the National Institute on Aging.

Their numbers are projected to grow at more than 20 times the rates of the total population by 2050, making them the fastest growing age segment.

Demographers attribute booming long-livers to decades of medical advances and improved diets, which have reduced heart disease and stroke. Genetics and lifestyle also play a factor. So, too, do doctors who are more willing to aggressively treat the health problems of people once considered too old for such care.

“My parents are 86 and 87 and they’re going strong, with my dad driving all over the place, so I’ve already told my financial planners that I’m going to live to at least 96,” said Susan Ryckman, 61, as she walked around New York City, an iPod and iPhone in hand.

“As long as I’m not mentally and physically infirm, I’d like to live as long as I can,” she said.

Japan, known for its low-fat staple of fish and rice, will have the most centenarians in 2050 — 627,000, or nearly 1 percent of its total population, according to census estimates.

Japan pays special respect to the elderly and has created a thriving industry in robotics — from dogs and nurses to feeding machines — to cater to its rapidly aging population.

Italy, Greece, Monaco and Singapore, aided by their temperate climates, also will have sizable shares of centenarians, most notably among women.

In the U.S., centenarians are expected to increase from 75,000 to more than 600,000 by midcentury. Those primarily are baby boomers hitting the 100-year mark. Their population growth could add to rising government costs for the strained Medicare and Social Security programs.

“The implications are more than considerable, and it depends on whether you’re healthy or sick,” said Dr. Robert N. Butler, president and chief executive of the International Longevity Center, a New York-based nonprofit group specializing in aging. “Healthy centenarians are not a problem, and many are. But if you have a demented, frail centenarian, they can be very expensive.”

Butler predicted a surge in demand in the U.S. for nursing homes, assisted living centers and other special housing, given the wave of aging boomers who will be at increased risk for Alzheimer’s disease. He said federal and state governments may have to re-evaluate retirement benefits, age limits on driving and Medicare coverage as they struggle to redefine what it means to be old.

“We don’t have a major coordinating figure such as a White House counselor to reach across all departments, and we need one,” Butler said.

Wan He, a Census Bureau demographer who co-wrote the aging report, said families also will face more pressure. She noted that because of declining birth rates, there will be fewer family members to provide support if an older parent gets sick.

“For the current middle-aged people, it will be comforting to think they can live past 80,” she said. “At the same time, we might see 70-year-old ‘kids’ taking care of their centenarian parents. It’s a very stressful job, it’s not paid, and it can have a lot of psychological influences for the caretakers.”