NORTHBROOK, Ill. >> Matea Peric knew her optometrist was elderly.

He walks slowly, and his face is grooved under salt-and-pepper gray hair.

She thought maybe he was 80 years old? 90? But she found out one day, while talking with the office’s receptionists, that her estimate was a bit off.

Dr. Daniel Nast turned 100 in February.

Yet twice a week, his wife still drives him to and from the Mind-Eye Institute in Northbrook where he sees patients, continuing a career that started about 75 years ago. He works at another practice on Saturdays.

“I don’t think it’s unusual,” Nast said of continuing to practice now that he’s a centenarian. “But everyone else does.”

To be sure, optometrists tend to be an older group. Nearly 44% of licensed, practicing optometrists are age 50 or older, with about 5% age 70 or older, according to the American Optometric Association. Still, the association’s president, Dr. Samuel Pierce, said he’s never heard of another 100-year-old practicing optometrist.

In the medical profession, the question of at what age doctors should retire has been a controversial one in recent years.

But Pierce said he doesn’t see why an optometrist shouldn’t be able to keep practicing at any age, “as long as they stay current in technology and education and training.” And if an optometrist can do all that, he said, “I think it’s amazing and truly something to be proud of.”

Nast still reads research articles voraciously, and while he’s shunned email (“I suppose I should,” he said, “but I don’t feel like I want to at this point in my life”) he works to keep up with his profession’s technology. Much has changed since Nast first began in the field — soft contact lenses hadn’t yet been invented when he began his career — but some things have remained the same.

At a recent appointment, Nast, clad in a crisp white shirt, sat next to the 20-year-old Peric, running through exercises designed to improve her visual processing and nearsightedness. He asked her to hold the end of a string, studded with colored beads, paying attention to different beads. He asked her to peer into a telebinocular, where two pictures merged into one.

And he had her sit in front of a large touch screen, listening for words and then tapping them with her fingers. Though he spent most of the appointment sitting, he easily manipulated equipment and walked across the room, unassisted, when necessary. Nast himself uses reading glasses on occasion.

“I think he’s absolutely nuts to be working,” joked Nast’s son, Richard Nast. But the younger Nast, an optician in Glenview, understands why he does it. His dad always told him and his sister that “if you like what you do, you never go to work.”

“He’s passionate about what he does and that’s why he’s been doing it so long,” Richard Nast said. “Being 100 years old, I think that’s what’s keeping him going, is working.”

The elder Nast discovered optometry as a boy, growing up in the small town of Little Falls, N.Y., the youngest of three children. As an 8- or 9-year-old, words would blur before his eyes, and he couldn’t grasp their meaning. He met with an optometrist who did visual therapy with him, and he saw improvements.

“It left an impression on me, that here is something worthwhile,” Nast said.

He held onto that thought as he searched for a career as a young man. Ultimately, he departed Little Falls, hungry for a faster pace of life. He sat up all night on the train ride to Chicago.

“I got to Chicago with a suitcase and $7 and that was it,” Nast said. “I was an 18-year-old kid and naive as anybody could possibly be, didn’t know anything about anything.”

He got a job sweeping floors and eventually started attending school at night. He thought he’d be an accountant but found the work too mechanical. He remembered optometry and decided to study it instead.

He was just shy of graduating when, amid World War II, the Army came calling. To make sure he’d have enough time to finish his courses and take his exam, he joined the Army Air Forces. The Army wanted to make him a bombardier, a navigator or a mechanic, but Nast told them he’d be more useful as an optometrist for military men.

Nast introduced himself to the leaders of the base’s eye clinic. Soon he was working as an optometrist on the base near Wichita Falls, Texas, where he stayed for three years.

Just before he was discharged, he was transferred to serve as an eye doctor on the base at Los Alamos, N.M. — the site where the atomic bomb was developed. He passed his evenings playing bridge with the scientists who were working on the history-making bomb.

“We would sit in the evenings because there was absolutely nothing to do, and you couldn’t leave,” Nast said.

After the war, he opened his practice in Melrose Park, got married and started a family. His wife, Phyllis Nast, died in 1985. He and his second wife, Dolly Howard- Nast, 90, have now been married for more than 30 years.

He continued to operate his practice in Melrose Park for about 50 years, until he sold it around 2000. Nast gave retirement a try. It didn’t stick.

“I almost went crazy,” he said.

Instead, he spent years “freelancing,” as he calls it, working at various doctors’ offices, pitching in when needed. About five years ago, Dr. Deborah Zelinsky, founder of the Mind-Eye Institute, an optometry office with an emphasis on visual rehabilitation and neurodevelopment, hired him as a permanent, if part-time, employee. These days, he mostly does visual therapy.

“Seventy years of experience is a wealth of knowledge,” Zelinsky said. “People come back and tell me how wonderful he is, that he understands them, that he listens to them.”

Nast has no plans to try retirement a second time.

“I work because I feel I’m doing some good, No. 1. And, No. 2, I enjoy it. It’s not work as far as I’m concerned,” Nast said. “I hope to be able to work as long as I’m able to, because this is what it’s all about, is helping people.”