(Anindito Mukherjee/Bloomberg News)

For centuries, education has been seen as something that happens to young people. A college degree, in particular, is often considered a once-in-a-lifetime experience, typically completed in one’s early 20s. As a result, most U.S. colleges focus on recruiting some 2 million teenagers who graduate from high school annually, even though there is a much larger market of 80 million American adults who dropped out of college or never went at all.

This traditional view about the life span of higher education is slowly beginning to change. One reason is that college graduates are realizing that the one-and-done approach to education after high school is no longer suitable for a workplace in which the skills to keep up in any job are churning at a faster rate. A survey out this week from LinkedIn of Generation Z in the workforce — those in their early to mid-20s — found that 20 percent have already had four or more jobs in the short time they have been out of school.

The other reason attitudes are shifting is that for workers without any credentials beyond a high school diploma, the economy is demanding additional education. Two-thirds of jobs in the United States will soon require education or training after high school, according to Georgetown University’s Center on Education and the Workforce. That doesn’t necessarily mean a four-year degree — which is the vision of life after high school for many Americans. Associate degrees and certificates are in greater demand, according to the Georgetown center’s analysis of jobs paying roughly $40,000 a year that don’t require a bachelor’s degree.

Those jobs are often referred to as “middle-skill jobs,” which require more than a high school diploma but less than a bachelor’s degree. The nonprofit National Skills Coalition calculates that middle-skill jobs — in computer technology, health care, construction, advanced manufacturing and other fields — account for 54 percent of the labor market, but only 44 percent of the nation’s workers are sufficiently trained to middle-skill level.

Many of those jobs are filled by aging baby boomers who will soon be retiring. It’s expected that as many as 25 million of all new job openings in the next decade will be for middle-skill jobs.

The problem is only made worse because, like almost any job these days, the skills of middle-skill positions are constantly evolving. Take, for example, a technical job repairing smartphones. When a new phone model is released, those skills usually need to be updated. The same is true for software installers. But unlike professionals who might have multiple degrees, workers in middle-skill jobs sometimes lack what education experts refer to as a “growth mind-set.” In other words, they lack the flexibility to learn new things.

Often, that’s because traditional middle-skills education is focused only on “vocational and technical skills” but doesn’t include soft skills, said Stephen M. Kosslyn, a former dean of social science at Harvard University and an expert in learning science.

I first met Kosslyn in 2015 when he was the chief academic officer at the Minerva Project, a for-profit company that is building a new kind of elite liberal-arts education experience. Now, Kosslyn has taken his decades of research on how people learn and is developing a school to teach the skills needed for middle-career jobs.

Backed with $6 million in venture capital, Kosslyn plans to launch Foundry College in January. It’s an online school aimed at working adults who need a new job or want to be promoted because of advancing technology. What makes his curriculum different from traditional training programs or technical degrees, he told me, is that it is designed to teach students both the cognitive and emotional skills — such as critical thinking, problem solving, communication — that people need to evolve in jobs.

The college’s classes will be synchronous, meaning in real time, and offered in the morning, during the day and in the evening to accommodate work schedules. All the work will be conducted in 90-minute classes twice a week, because as Kosslyn said, “very rarely do students complete readings before lectures.” Two-thirds of class time will be spent working in small groups solving problems or working on projects.

Students who complete the 15-week courses will receive certificates, and the plan is to eventually award associate degrees to those who complete 12 courses. The courses all focus on what Kosslyn called “business management” to prepare people to oversee technology groups, run a franchise or work in business operations of a hospital.

Kosslyn is far from the only educator or entrepreneur trying to create pathways to middle-skill jobs. In recent years, “boot camps” have started to offer short courses that help workers gain in-demand skills, such as computer coding. Many community colleges are beefing up their offerings of noncredit courses that provide skills for jobs. And the continuing education divisions of traditional four-year colleges continue to add certificate programs to appeal to workers interested in middle-skill jobs. Despite these efforts, millions of middle-skill jobs go unfilled every year, and fewer than half of U.S. adults have a post-high-school credential that is of value in the economy, the Lumina Foundation, a nonprofit group focused on higher education, has found.

Part of the issue is that workers who feel left out of this fast-moving economy often don’t know of options to gain new skills. And for the most part, as a nation, we have failed to legitimize anything that is not a four-year college or focus on finding a pathway to further education for adults who dropped out of college. Unless we consider education as something that happens for life and add more training opportunities for workers at all stages, the education gap among voters that emerged during the 2016 presidential election will only widen.