In 2010, according to Gallup, 40 percent of 65-year-olds were still in the workforce to one degree or another. By last year, the percentage was the same.

Its figures, Gallup said, are based on 134,168 interviews conducted in 2014 with Americans aged 50 to 64, those born from 1946 through 1964, the post-World War II years commonly used to define the baby boom generation. Respondents were considered as being "in the workforce" if they were employed full-time or self-employed, worked part-time, or out of work but able to work and looking for  employment.

"The 2010 to 2014 comparison is important because it provides a valuable look at the first group of boomers who have moved past the traditional retirement age of 65 over the past four years, and the results do not support the hypothesis that baby boomers are staying in the labor force longer than those who came immediately before them," Gallup said.

The aging of the U.S. population, Gallup noted, leaves a smaller percentage of the population working and "supporting their elders through Social Security and Medicare." 

It also means the U.S. could face a shortage of workers without a substantial influx of people from other countries, it said. 

Another Gallup survey, the result of which it reported last spring, found that the average age at which U.S. retirees report retiring is 62. From 2002 through 2012, the average had hovered around 60. The increase was attributed to the economic downturn and inadequate savings.