A December US Census Bureau report found that 39 percent of people over age 65 battle at least one disability, and the most common hurdle they face is serious difficulty walking or climbing stairs. The ranks of older, disabled adults could swell rapidly, the report cautioned, as the number of aging baby boomers increases. Such disabilities mean more Americans losing independence and relying on costly, long-term health services. Which is why researchers in Boston and across the country are redoubling efforts to pinpoint the elders most likely to become disabled, and crafting simple exercise programs to stem the downward slide. Jonathan Bean, associate professor of physical medicine and rehabilitation at Spaulding Rehabilitation Hospital, is leading research on the subject.