Joined by graying baby boomers, the oldest old are projected to increase to 8.7 million by midcentury — or one in 10 older Americans. That’s a big change from over a century ago, when fewer than 100,000 people reached 90.

Analysts attribute the increases in the 90-plus age group mostly to better nutrition and advances in medical care that have reduced heart disease and stroke. Still, the longer life spans present a fresh set of challenges for disabilities and chronic conditions such as arthritis, diabetes and Alzheimer’s disease.

"A key issue for this population will be whether disability rates can be reduced," said Richard Suzman, director of behavioral and social research at the National Institute on Aging, which supported the report.

Figures show that smaller states had the highest shares of their older Americans who were at least 90. North Dakota led the list, with about 7 percent of its 65-plus population over 90. It was followed by Connecticut, Iowa and South Dakota. In absolute numbers, California, Florida and Texas led the nation in the 90-plus population, each with more than 130,000.

Traditionally, the Census Bureau has followed established norms in breaking down age groups, such as under 18 to signify children or 65-plus to indicate seniors. Since the mid-1980s, the bureau often has released data on the 85-plus population, describing them as the "oldest old."

But some of those norms, at least culturally, may be shifting. Young people 18-29 more than ever are delaying their transition to work in the poor job market by pursuing advanced degrees or moving in with Mom and Dad. Older Americans, who are living longer and staying healthier than prior generations, are now more likely to work past 65.

On Thursday, the Census Bureau said it was putting out its study of the 90-plus age group at the request of NIA in recognition of lengthening age spans.

"Given its rapid growth, the 90-and-older population merits a closer look," said Wan He, a Census Bureau demographer who wrote the report. "The older people get, the more resources they consume because of health care, and disability rates significantly increase. This creates demands for daily care, and for families the care burden increases dramatically."

According to the report, the share of people 90-94 who report having some kind of impairment such as inability to do errands, visit a doctor’s office, climb stairs or bathe is 13 percentage points higher than those 85-89 — 82 percent versus 69 percent.

Among those 95 and older, the disability rate climbs to 91 percent.

The findings come as a special congressional committee struggles to meet a Nov. 23 deadline to cut more than $1 trillion from the federal deficit over 10 years. Major sticking points are proposals to increase tax revenue as well as trim Social Security and Medicare spending, such as by increasing the Medicare eligibility age.

Other findings in the census report:

—Among the 90-plus population, women outnumber men by a ratio of nearly 3 to 1.

—Broken down by race and ethnicity, non-Hispanic whites made up the vast majority of the 90-plus population, at 88.1 percent. That’s compared to 7.6 percent who were black, 4 percent Hispanic and 2.2 percent Asian.

—Most people who were 90 or older lived in households alone, about 37.3 percent. Another 37.1 percent lived in households with family or others, while about 23 percent stayed in nursing homes. About 3 percent lived in assisted living or other informal care facilities.

—Those who were 90 or older had median income of $14,760, about half of which came from Social Security. About 14.5 percent of the age group lived in poverty, compared to 9.6 percent for Americans who are 65-89.

A 2009 Pew Research Center poll found that Americans, on average, would like to live to 89; the current life span is roughly 78. One in five people said they would like to live past 90, while 8 percent would prefer to pass 100.

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Online

National Institute on Aging: http://www.nia.nih.gov/

Census Bureau: http://www.census.gov/