There is a Bulgarian doctor who still practices medicine well beyond retirement age.

Kiril Apostolov doesn’t seem to tire when walking along inclined streets in the mountain village of Skrat while carrying a medical bag. At age 90, this is unusual. In Bulgaria, many retired doctors continue to work well past retirement age because there is no one to replace them, especially in rural areas, Agence France-Presse (AFP) reported.

Apostolov’s list of patients numbers 1,500, including 88-year-old Milka Oumlenova, who uses a walking stick, unlike her doctor.

The old doctor starts work early — at 7 a.m. — driving roughly 16 kilometers from his home in Petrich to his clinic in Skrat.

“For the moment, I feel fine. The work doesn’t weigh me down. I’ll keep going as long as I’m doing well and the patients are satisfied,” he told AFP.

For scientists at the Skolkovo Institute of Science and Technology in Russia, the likes of Apostolov are not surprising.

The researchers, led by Dr. Dmitrii Kriukov, are studying biological processes that collectively contribute to aging, including how cellular behavior called somatic mutations gradually depletes cells across the body’s tissues, Yahoo! News reported.

If medical science invented a perfect anti-aging treatment that wiped out every hallmark of aging except somatic mutations, the researchers’ model estimated that average life spans would jump to between 146 and 194 years.

Around those ages, somatic mutations reach a critical tipping point, impairing normal cell function and degrading tissues, inevitably leading to organ failure.

The researchers are not offering a formula to live well beyond supercentenarian age, but their model maps out where future anti-aging treatments can be most effective.