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 six million by mid-century. 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 US and Japan, according to the latest Census Bureau figures and a report 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, helped by their temperate climates, also will have sizable shares of centenarians, most notably among women. In the US, centenarians are expected to increase from 75,000 to more than 600,000 by mid-century. 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 US 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 might have to reevaluate 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 less family members to provide support if an older parent falls ill. "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." Census estimates show: â€¢ Come 2017, it will be the first time there will be more people 65 and older than there will be kids younger than five. â€¢ The population of people 80 and older is projected to increase 233% by 2040, compared with a 160% increase for people 65 and over and 33% for the total population of all ages. â€¢ Childlessness among European and US women age 65 in 2005 ranged from less than 8% in the Czech Republic to 15% in Austria and Italy. About 20% of women 40 to 44 in the United States in 2006 were childless. â€¢ Due to low birth rates, Japan's median age will increase from 37 in 1990 to 55 by 2050. The median age for the world during that same time will rise from 24 to 37, slowed by younger populations in Latin America and Africa. â€¢ The median age in the US will edge higher from 33 to 39, kept low by higher rates of immigration. In the US, experts say rising rates of obesity for people who are more sedentary or eat too much junk food could take a toll on life expectancy. AARP and other groups are trying to promote healthier lifestyles. AARP is conducting a 10-month pilot project in Albert Lea, Minnesota, aimed at extending the life span of residents by two years. The group is with working with the city to make easier to get around on foot or bike, develop social networks and provide healthier fast-food options, and is hoping to expand the effort to other cities. A recent Pew Research Center poll of 2,969 adults found that Americans, on average, would like to live to 89; the current life span is 78. One in five people would like to live past 90, while 8% would like to pass the century mark. "Our motto is that dancing boomers are forever young," said Julie Dahlman, 62, co-founder of a 300-member boomers social club in Portland, Oregon, that hosts dances, golf outings, hikes and wine tastings. Dahlman said that after caring for a 92-year-old mother with Alzheimer's, she knew it was important to live life to its fullest. "I'm silly with my girlfriends, and we still have a slumber party once in a while," Dahlman said. "We're not going to go away quietly."