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.

“Community colleges just don’t have the money to expand,” she said. “At the for-profits, every person who comes there they’re making money on, so boy, are they expanding.”

The increase in college degrees is likely to fuel a debate about the wisdom of having so many people flock to college, given high debt levels and stories of unemployed graduates who are stuck on their parents’ couches.

Many economists point out that college graduates have fared much better than their less-educated peers and argue that rising educational levels will help the economy in the long run. Since the recession began in December 2007, the number of Americans with bachelor’s degrees who have jobs has risen by 9 percent, while employment has fallen for everyone else.

The unemployment rate for graduates of four-year colleges between the ages of 25 and 34 was 3.3 percent in March, according to the Bureau of Labor Statistics. For high school graduates in the same age group who had not attended college, it was 11.8 percent.

Today’s premium for college degrees is caused partly by increasing selectiveness among employers about whom they hire and screening based on education even for positions that do not require higher skills. But jobs themselves have changed, too.

“Think about jobs 15 years ago that didn’t need any college education,” said Sandy Baum, a senior fellow at the George Washington University Graduate School of Education. Many of them now do, she added.

“Maybe you don’t need a bachelor’s to change bedpans,” Ms. Baum said, “but today if you’re an auto mechanic, you really have to understand computers and other technical things.”