YPSILANTI, Mich. — Earl Johnston was 12 when he got his first job, a paper route that he quickly expanded by “buying out” additional routes from other, less industrious boys.

After high school, he enrolled in a program to become an industrial electrician.

“With that journeyman’s card in my back pocket, I had never had trouble finding work — ever,” he said.

Like many people in the prime of their working life Johnston, now 56, always thought he would have plenty set aside for retirement, and if he didn’t, he could always work a little longer. “At least up to 65,” he thought.

What Johnston didn’t plan for was the degenerative spinal condition that made being on his feet for a 12-hour shift all but impossible.

“I could be making lots and lots of money right now, but nobody in manufacturing is going to hire a 50-something guy who can’t stay on his feet,” he said.

Johnston’s story might be chalked up to simple bad luck. But it serves as an example of the rapidly changing expectations that seniors have for being able to work well past traditional retirement age, and in some cases, dismissing the idea of retirement altogether.

According to the 2015 Employment Benefit Research Institute Confidence Survey, the percentage of workers aged 25 and over who planned on working past 65 has increased steadily from 11 percent in 1991 to 36 percent in 2015.

But those expectations aren’t always achievable.

The University of Michigan Health and Retirement Study found that 37 percent of respondents didn’t reach the retirement age they had set when they were 58.

“The first, and maybe the most obvious explanation people give for early retirement is health,” said Boston College research economist Geoff Sanzenbacher, co-author of a working paper investigating causes for this mismatch between retirement expectations and reality.