Carl Jensen experienced what he calls “the awakening” sometime around 2012.

He was a software engineer in a suburb of Denver, writing code for a medical device. The job was high-pressure: He had to document every step for the Food and Drug Administration, and a coding error could lead to harm or death for patients.

Mr. Jensen was making about $110,000 a year and had benefits, but the stress hardly seemed worth it. He couldn’t unwind with his family after work; he spent days huddled over the toilet. He lost 10 pounds.

After one especially brutal workday, Mr. Jensen Googled “How do I retire early?” and his eyes were opened. He talked to his wife and came up with a plan: They saved a sizable portion of their income over the next five years and drastically reduced expenses, until their net worth was around $1.2 million.

On Tuesday, March 10, 2017, Mr. Jensen called his boss and gave notice after 15 years at the company. He wasn’t quitting, exactly. He had retired. He was 43.