The recent jump in college graduation mirrors similar increases in educational attainment during previous severe downturns, economists said.

“It was sort of one of these ironic good things about the Great Depression, that it got all these kids to graduate from high school, which turned out to be really good for workers later on,” said Claudia Goldin, an economics professor at Harvard.

The G.I. Bill then created a second surge in educational investment after World War II, which also helped fuel the postwar economic boom. Of course, in those cases, Professor Goldin said, education was free or very cheap; college today is not.

Cost may be one reason that college completion has not risen nearly as much for low-income students, many of whom take on large amounts of debt and often do not graduate. The share of 24-year-olds from low-income families who hold college degrees has remained relatively flat over the last several decades, according to Tom Mortenson, a higher education policy analyst with Postsecondary Education Opportunity, a newsletter.

Low-income students are less likely to graduate from high school than more affluent students, less likely to enroll in college after high school and less likely to graduate from college after enrolling. Only about 1 out of 10 Americans whose parents were in the lowest income quartile held four-year college degrees by age 24 in 2011; the comparable share for people from the highest quartile was about 7 in 10, according to Mr. Mortenson.

Some of the recent increase in college completion has come among students who enroll in college, or return to it, at older ages, and experts say any future increases will probably need to come among this group as well, given its growth potential.

For-profit colleges — despite being more expensive and having lower completion rates than other colleges — are taking in many of these older and lower-income students. Professor Goldin estimates that for-profit colleges account for about one-fifth of the increase in bachelor’s degrees over the last decade.