Russell Pate was 25 with a wife, a daughter and one career already behind him when he began to work towards a degree in cardiovascular technology at Northeastern University in Boston.

"I felt like I was the oldest one in the bunch," said Mr Pate, most of whose classmates were 18 to 22-year-olds straight out of high school. He had already served four years in the United States Air Force but could not get the medical training he wanted because of military cutbacks. "I just wasn't in the field I wanted to be in," he said. "I figured it was time to continue my education."

After five years raising a family, working two jobs and taking courses, Mr Pate will graduate in June aged 30. These days more of his classmates are closer to his age.

Nearly half of all college students in the US today are over 24, up from fewer than one third in the 1970s, according to a new survey. Most have returned to school because of changes in their careers or to enhance their skills in an uncertain job market. Of those, 70 per cent are enrolled in degree programmes alongside younger students - not in special programmes or extension classes - changing the face of many campuses.

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If the trend continues, there will be 15 million students older than the traditional age enrolled on undergraduate and graduate programmes by the year 2000, the higher education association, the College Board, reported.

Colleges are "turning themselves inside-out" to accommodate this change, according to Carol Aslanian, board director of adult learning services.

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More and more are scheduling classes in the early morning, late evening and weekends and at satellite locations closer to their students' offices and homes. Still, most older students attend some courses during the daytime, increasing the average age on many campuses.

"This is a different population from your more passive 18-year-old who comes in and is more accepting and less demanding," Ms Aslanian said.

Most of these older students are changing jobs or trying to get ahead in their careers in an uncertain employment market; 90 per cent said they were trying to learn new skills because the nature of their work was changing, according to the survey.

Thirty per cent of older students said they attended school full-time while also juggling jobs and families.

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"People need to go back to school not only to advance in their jobs, but just to stay in place," said Ms Aslanian. "The nature of jobs and the work we do is changing under our feet. You can't stop learning. More and more people are vying for fewer jobs and the best-credentialed people tend to get the highest-paying jobs."

At the undergraduate level, older students usually study business, according to the survey. At the graduate stage, the most popular major is education, a growth industry because of an increase in the number of children being born.

"What the road ahead looks like is that people are going to be in and out of jobs all during their lifetimes," Ms Aslanian said. "They'll be going back to school for training while they're in and out of the job market."

Meanwhile, only 20 per cent of traditional-age university and college students finish their degrees in four years. Many stretch out their academic schedule so they can work part-time to pay tuition, or leave and then return to school. They also are contributing to the ageing of the campus.

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It has made the average higher education not only longer, but tougher. Just ask Mr Pate. When he finally gets his degree next month, he said, he's "going on a long, well-deserved vacation".