At 89, Donald Tenbrunsel is a bit of a phenomenon. He surfs the internet with ease, happily converses on a broad range of timely topics, volunteers and reads regularly.

Known as a “Super Ager,” Tenbrunsel was part of a study that helped researchers discover what factors might set these super-sharp seniors apart from their peers.

The secret? Brain scans showed they experience brain aging twice as slowly as average folks their age.

“This suggests the Super Agers are on a different trajectory of aging,” said senior researcher Emily Rogalski. She is director of neuroimaging for Northwestern University’s Cognitive Neurology & Alzheimer’s Disease Center. “They’re losing their brain volume at a much slower rate than average agers.”

For the study, Rogalski and her colleagues measured brain aging by examining the thickness of each person’s cortex — the outer layer of folded gray matter in the brain.

The cortex is where consciousness lies, and where all of the neurons that fire thoughts and movements are located. It is a critical part of the brain for higher-level thinking, memory, planning and problem-solving, Rogalski said.

Another neurologist explained it this way:

“That is essentially our brain,” said Dr. Paul Wright, chair of neurology at North Shore University Hospital in Manhasset, N.Y., and Long Island Jewish Medical Center in New Hyde Park, N.Y. “Brain shrinkage occurs in the natural progression over time, and when you lose brain volume, you lose function.”