BELFAST, Maine — Some of Aaron Sarnacki’s computer service clients have been with him 10 years. Not only is such customer loyalty unusual in the rapidly changing industry, but even more remarkable is that Sarnacki is just 23.

A 2006 graduate of Searsport District High School, where he was valedictorian, and a 2010 graduate of the University of Maine, Sarnacki planned a career as a physician. He earned a bachelor’s degree in biology with a minor in pre-med.

But a funny thing happened on the way to medical school — his part-time business, Archangel Computers, grew to the point that he was ready to open storefronts in both Belfast and Camden. The Belfast location, on upper Main Street near Weaver’s Bakery, opened for business earlier this month. The Camden store, on Elm Street at the corner of Washington Street, will open next month.

On Monday afternoon, Sarnacki fielded a question from one customer about installing a wireless printer. A few minutes later, another customer asked about Web design help. A third started perusing the Apple laptops he has for sale, displayed on a counter.

“We can pretty much do anything,” Sarnacki said, including repairing computers that have suffered hardware or software failures, removing viruses, installing software, networking computers for businesses and providing IT support on an on-call basis.

After working out of his family’s home in Searsport, Sarnacki last summer set up shop in a building across from City Hall on Church Street. Business was good enough that he was ready to take the storefront plunge.

“We support hundreds of businesses in the area,” he said.

Many of Archangel’s clients are physician offices; Sarnacki and his two full-time employees help them set up electronic medical records. Some area public libraries have outsourced their IT work to Archangel. He also works with high-end digital photographers and computer-assisted designers.

So how does a young man grow a business so effectively?

“It’s just word of mouth,” Sarnacki said. “Once we get a customer, we don’t lose them.”

Archangel repairs both PCs and Apples and sells new and refurbished Apples and Dell PCs, as well as accessories and parts, even really hard-to-find stuff, Sarnacki said. Gesturing to the various Apples displayed on the two counters that line the Belfast store, he said, “We’re the only place in the midcoast where you can find this stuff north of Portland.”

Sarnacki renovated the Main Street storefront in a clean, hip style: unpainted steel ceiling panels in the 1890s fashion, dark-stained composite floor and art lights, all in a minimalist setting. The repair work is done in the back.

Archangel is open 9 a.m. to 5 p.m. Monday to Friday and 10 a.m. to 4 p.m. Saturday. The business’s on-call number is 249-8054.