The 20 million young people in this country who are not likely to attend college face much more difficult lives than their counterparts in the past because of fundamental changes in the economy, a commission of business leaders and educators reported yesterday.

In the first comprehensive examination of the "non-college youth" who make up about half of the nation's 16- to 24-year-olds, the commission, funded by the William T. Grant Foundation, reported on the decline of stable, well-paying jobs not requiring advanced training. It said 1.7 million manufacturing jobs disappeared between 1979 and 1985 and added that these young people are forced to "scramble for unsteady, part-time, low-paying jobs."

Dubbing this group "the forgotten half," the commission said: "The plight of the 'forgotten half,' never easy, has become alarming. This nation may face a future divided not along lines of race or geography, but rather of education."

In 1985, for example, only 44 percent of all men aged 20 to 24 earned enough to support a family of three above the poverty line, compared to 60 percent in 1973, according to the study. And in 1986, male high school graduates in this age group who did not go on to college were earning an average of 28 percent less in constant dollars than a comparable group in 1973. The drop was 44 percent for blacks, the study said.

The 19-member commission, headed by Harold Howe II, former U.S. commissioner of education, focuses on the broad repercussions of the elimination of millions of well-paying jobs in manufacturing, transportation and agriculture once available to men and women without higher education.

The primary problem lies not with the young people, the report said, but with the economy. Millions of new service and retail jobs -- the positions open to people without higher education -- pay wages at half the level of manufacturing jobs.

In addition to the slide in earning power, there are also more young people who report no earnings. In 1973, about 7 percent of all 20- to 24-year-old men reported no earnings, but that number climbed to 12 percent by 1984, the study said.

"I don't think the country has any realization of what these kids are up against," Howe said. "They are really floundering."

The commission's report, part of a $1.5 million, two-year study, called for a $5 billion annual increase in federal spending over the next 10 years, primarily for programs it deemed successful, including the Head Start preschool program, federal aid for disadvantaged children, Job Corps and Job Training Partnership Act.

The group urged that state, local and private agencies attempt to ease the transition between school and work, suggesting internships, apprenticeships, pre-employment training and programs that would make it easier for adults to return to school.

Not all of the commission's findings were negative. Statistics show that more students are staying in school longer and, like older Americans, they aspire to succeed, the report said.