Too often, doctors resort to extrapolation — or, to put it less politely, guessing.

So the sodium survey published in the journal JAMA, which an accompanying editorial praised as “a major advance,” stung. “If the C.D.C. excludes old people, how do we tell drug companies not to?” Dr. Covinsky asked.

Asked to explain, the C.D.C. said the study excluded older people because they were less likely than younger adults to participate in 24-hour urine collection, which would have created an unrepresentative sample.

Critics of age exclusion had reason to celebrate in December, when the National Institutes of Health issued new policy guidelines for the research it funds.

Starting next January, grant applicants will have to explain how they intend to include people of all ages, providing acceptable justifications for any group they leave out. The agency will monitor investigators to make sure they comply.

“It’s the right starting point,” said Dr. Florence Bourgeois, a pediatrician at Harvard Medical School. (Children also wind up taking drugs tested only in adults.)

Yet she and other researchers point out that many studies are privately funded and thus will not be affected, and that merely eliminating age limits doesn’t mean older adults will actually be enrolled in studies.

“It really requires a cultural shift in our thinking,” Dr. Bourgeois said.

How often are old people left out of important medical research? In 2011, it looked like progress when Dr. Donna Zulman and her colleagues at the University of Michigan reviewed 109 clinical trials published in leading journals and found that just 20 percent set upper age limits for participation.