Sixty-six percent of American 4-year-olds are enrolled in early childhood education, placing the United States well below average compared to other developed countries at a time of increasing focus on early learning, according to a report released Tuesday.

On average, enrollment figures for 3-and-4-year-olds went up considerably in the countries surveyed for the report by the Organization for Economic Cooperation and Development. In 2013, an average of 88 percent of 4-year-olds in the countries were enrolled, compared with 72 percent in 2005. For 3-year-olds, the average enrollment went from 52 percent in 2005 to 74 percent in 2013.

But enrollment figures for American 3- and 4-year-olds didn't change much from 2005 to 2013. In the U.S., 41 percent of 3-year-olds were enrolled in 2013, compared to 39 percent in 2005.

"There is increasing awareness of the key role that early childhood education plays in the cognitive and emotional development of the young," the report said. "As a result, ensuring the quality of early childhood education and care has become a policy priority in many countries."

The "Education at a Glance" report, which analyzed education data from the 34 OECD countries, along with several partner countries, found that 15-year-old students who had at least a year of pre-primary education did better on an OECD international assessment test than those who did not.

More schooling later on paid off, too, according to the report by the international economic organization.

On average, more than 80 percent of college-educated adults were employed, compared with less than 60 percent of adults who didn't go beyond high school, and they earned about 60 percent more, the report said.

"In general, employment rates and earnings increase as an adult's level of education and skills increases," it said, "but the labor market still regards a diploma or degree as the primary indication of a worker's skills."

Among other findings:

— The United States, Canada, Australia, France, Germany and the United Kingdom together attract more than half of all international college students worldwide, but the United States' share is shrinking.

—International students made up 32 percent of students in U.S. doctoral or equivalent programs in 2012, but only 3 percent of students pursuing a bachelor's degree.

—The teaching force is aging across OECD countries. In 2013, 36 percent of secondary school teachers were 50 or older, up 3 percentage points from 2005.

—For all levels of education combined, 25- to 64-year-old women in the United States earned 73 percent of what men earned in 2013, below the OECD average of 80 percent.

—On average, OECD countries spent $8,247 per primary student per year and $9,518 on high school students in 2012. U.S. average spending was $11,030 and $12,442, respectively.