Tom Cooper, 85, leaves Nordstrom at Montgomery Mall after finishing his shift. (Sarah L. Voisin/The Washington Post)

When Tom Cooper came out of the Navy and got his first job selling shoes, Harry Truman was president and a gallon of gas cost around 18 cents. Now 85, Cooper still puts on a tie five days a week and drives from his home in Silver Spring, Md., to work as a shoe salesman, and he has no plans to stop.

One reason is that he can’t afford to — he needs money to pay bills from cancer treatment, a back operation and a foot operation. But even if he had enough savings to retire, he doubts he would.

“I’d go nuts. I’d have nothing to do,” he said. “I can’t lay around the house, do nothing and watch TV. I feel a lot better when I’m walking and helping people and doing stock work.”

Cooper is part of a small but growing segment of Americans who remain in the workforce into their 70s, 80s and 90s. Although the average retirement age for Americans is 63, the portion of people 75 and older in the workforce has more than doubled since 1985 — from 3.6 percent to 8 percent, according to Bureau of Labor Statistics data analyzed by the Integrated Public Use Microdata Series (IPUMS) at the University of Minnesota. For those working full-time, the increase has been even more dramatic: From just over 1 percent in 1993 to nearly 4 percent last year.

A major reason is that 75 no longer represents the very extreme end of life it once did, as life expectancy in the United States has steadily risen over the past century.

“We’re living longer lives, and we’re also living healthier lives, and that’s allowing a lot of us to work into our 70s and 80s, not because we have to, but because we want to,” said Linda Sharkey, co-author of “The Future-Proof Workplace,” a book on helping businesses prepare for the future. “We have a president who’s 70 years old, and by all accounts, I guess he thinks on his feet.”

People who put off retirement the longest tend to be in the highest and lowest-wage brackets, said Jen Schramm, senior strategic policy adviser at the AARP Public Policy Institute. The high earners tend to have more flexible schedules, love their work and have longer life spans in general, she said, while the low earners tend to stay at their jobs because of financial necessity.

The upward trend in the working old is actually a shift back to an earlier paradigm, said Steven Ruggles, a professor of history and population studies at the University of Minnesota.

Most workers in the 19th century were self-employed and generally worked as long as they needed to and were physically able to, he said. “In 1936, the introduction of Social Security made it financially more possible for a lot of people to retire, and that steadily expanded until the 1980s.”

At the same time, the proportion of wage-earning jobs increased after the Industrial Revolution, and many employers began mandating retirement at certain ages.

Then the 1978 Age Discrimination Act outlawed compulsory retirement before age 70 for most jobs, and over the following decade, the long decline in labor participation of people 75 and older bottomed out and began to climb.

Today, for men who remain in the workforce past 75, top occupations include chief executives and managers, farmers and ranchers, clergy, teachers, lawyers and judges, maintenance workers and real estate agents, according to IPUMS data. For women over 75, top jobs include elementary and middle school teachers, nurses, cooks, maids, personal aides, receptionists, cashiers and retail workers.

That broad range for the working old reflects a shift away from the idea that they can get hired only for “old people’s jobs,” such as substitute teachers, said Matthew Rutledge, a research economist at the Center for Retirement Research at Boston College.

“Given how young 75 seems now, I don’t think there’s quite as much discrimination and prejudice toward people who are looking to extend their career,” he said.

“There are more baby boomers in the HR department, so they’re going to be more positively disposed to looking at résumés from older people. There’s still age discrimination, of course, but it’s a little less grim than it used to be.”

Continuing to work might help people stay healthy. Studies show better cognitive abilities associated with working later in life. And jobs with more flexible schedules or where the worker is his or her own boss can be easier to keep into old age.

John Hays at The Phoenix, his family-owned boutique, founded by his parents. (Samantha Hays-Gushner)

John Hays, 75, runs The Phoenix, a shop in Georgetown that sells clothing, jewelry and folk items from around the world. He works four days a week and twice a year embarks on shopping trips abroad. His wife stopped working a year ago, but he has not felt the call. “I plan to do it as long as I enjoy it,” he said. “It’s energizing.”

Not all the jobs are cushy. Dorothy Zehnder, 95, works six days a week as the kitchen manager for the Bavarian Inn, in Frankenmuth, Mich. She has worked in restaurants, mostly the same one, since she was 16 — when F.D.R. was president — and she used to figure she’d retire around 65.

That plan lasted only two weeks.

“I was bored,” she said of her attempt to retire. “I said, ‘I think I’ll go back and fry chicken,’ and I did that.”

Dorothy Zehnder, 95, has been a restaurant worker since she was 16. (AMB Photography/Courtesy of the Bavarian Inn Restaurant)

Zehnder no longer peels potatoes or cuts cabbage, but she starts each day at 9 a.m. inspecting produce, and she works six to nine hours a day. On Mondays, her day off, she bakes at home.

Working has helped her stay healthy, she said. Occupied all day, “you forget, sometimes, your little ills and aches. You kind of forget when you’re around people and you talk to people, and when you hear their story, you think, ‘Oh, I’m a lot healthier than that.’ ”

In some fields, such as coaching or consulting, gray hair can be an asset. “You have to have experience in taking an organization through culture change, and that comes with a lot of time,” Sharkey said.

Age has improved the performance of Jennifer Girard, 74, a photographer in Chicago who has held the same job for 44 years. She no longer does certain physically demanding assignments, such as eight- to 10-hour weddings. But age has made her looser and more relaxed in her work.

“It’s great, because I don’t care who I shoot. If it’s a famous celebrity, there’s no heartbeat, there’s no anxiety — the kind of things I used to feel years ago. I technically know what I’m going to do … I’m an older soul now.”

In some professions, however, advanced age can raise questions of competency and legal liability. For certain jobs that involve public safety — such as pilots, air traffic controllers, federal law enforcement officers, national park rangers and lighthouse operators, Congress mandates fixed retirement ages.

Doctors, however, have no cutoff date, and it can sometimes be difficult to determine when they are no longer able to do such things as prescribe medications or perform surgery.

Self-regulation does not always work, says Mark Katlic, chairman of the surgery department at Sinai Hospital in Baltimore.

“The reality is that all human faculties deteriorate with increasing age,” he said. “Surgeons don’t like to think they’re human, but they are.”

Citing studies showing poorer outcomes among older surgeons for certain procedures, he added: “Everyone knows someone who should have stopped practicing surgery before he did. It is a pervasive problem.”

Because older people’s cognitive and physical abilities vary widely, Katlic doesn’t advocate a blanket age cutoff for surgeons, but rather a test to make sure their faculties are intact.

Since last year, all health practitioners 75 and older who apply for a new credential or recredentialing at his hospital must take a general physical, eye and cognitive exam.

In 2014, Katlic started the Aging Surgeon Program, which offers a two-day extensive screening for older surgeons. Two people have gone through it, but he said that more than a dozen have voluntarily retired after being threatened with the program.

State Sen. Fred Risser of Wisconsin has no such concerns: At 89, he is the country’s longest-serving legislator and was recently elected to another four-year term. Although his wife retired 15 years ago, he has no such plans.

“My dad never retired, and my grandfather — they kept going. Retirement was never anything that I anticipated or looked forward to,” he said, adding that he also has kept his law office open while in office.

“It keeps the adrenaline going. … It keeps you in better health — it keeps you active. I learn something new every day.

“If you’ve got a good life,” he said, “why throw it out?”