WASHINGTON: It's starting to get crowded in the 100-year-olds' club.

Once virtually non-existent, 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-aged.

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 released yesterday by the National Institute on Aging.