They may be a little bit older, but they are still sharp as a tack...group of "SuperAgers" volunteer for a study of the aging brain.

CHICAGO—As soon as the crowd of chatty octogenarians and nonagenarians shuffled away from the bar, Alex Karczmar, wearing brown-and-white wingtips, sidled over. It was 4:30 in the afternoon, and the 96-year-old was ready for another glass of red wine.

Mr. Karczmar and the others are part of a group of 42 people over the age of 80 identified by Northwestern University researchers as having excellent cognitive function and memory capability typical of those decades younger. The researchers have been studying the group of so-called SuperAgers for years, but a party this week at the medical school's downtown campus was the first time they'd all been brought together.

As a jazz trio played Gershwin classics, the partygoers sipped cocktails and chatted about their interests and role in the study. "Evolutionarily speaking, we're supposed to get to 30, put some babies in the world and then perish," said Mr. Karczmar, a retired neuroscientist who still attends professional conferences. People aren't designed to reach such advanced age, he said, and those who do should contribute to research—if for no other reason than to offset their pension and medical bills. "Can you imagine how much I cost the country?" he asked, laughing.

Researchers at Northwestern's Feinberg School of Medicine identified an original group of SuperAgers six years ago when they noticed that some participants in an Alzheimer's study didn't even exhibit the memory loss common in people in their 80s and beyond. Dr. Emily Rogalski and her colleagues in the school's Cognitive Neurology and Alzheimer's Disease Center wanted to find out why.

"When we think about aging, so many of the stories are about what goes wrong," said Dr. Rogalski, one of the lead researchers. "Things just go down on average...and yet, there are people who don't seem to lose it after 80."

Researchers gave a battery of tests to prospective SuperAgers, identifying those who performed much better on basic memory tests than their peers. In MRI scans, the SuperAgers' brains also showed physical differences compared with their peers.

Average brains as they enter advanced age become increasingly prone to clogs in their microscopic plumbing. People with Alzheimer's have an abnormally large number of such blockages. SuperAgers' brains, by contrast, have "fewer plaques and tangles," said Dr. Changiz Geula, another lead researcher. And while the cortex—the wrinkly outer part of the brain responsible for much thinking and memory—generally shrinks with advanced age, MRI images of SuperAgers' cortexes show no such atrophy.

Although researchers say many variables can contribute to such brain health, including good blood flow and lack of trauma, like concussions, they don't know exactly what leads to above-average brain function. But the Northwestern researchers are getting a better idea of what an exceptionally healthy brain looks like and are beginning to figure out exactly why some remain that way.

"The Northwestern study is critical," said Maria Carrillo, a vice president at the Alzheimer's Association, a support and research group. "It demonstrates that healthy aging can occur." The study validates findings from other research, but it also is starting to shed new light on questions like why the brains of people with dementia and Alzheimer's undergo physical changes—and ultimately could help more people maintain mental health in old age.

The program is still accepting participants, but only about 10% of those who think they have an outstanding memory and volunteer pass the required tests, said Dr. Rogalski. The 42 current participants come in every 18 months for a three-day battery of tests.

The lab also maintains a "brain bank" of brains donated by people who have died, which is important for research because there are limits to what MRIs can show. Dr. Rogalski said she constantly fights for more funding, much of which comes from the university and a grant from the Davee Foundation, a Chicago-based nonprofit.

Partly because of funding concerns, the program draws most of its participants from the Chicago area. That's not optimal for research because it limits diversity among participants, but it made it easier to gather most everyone for Tuesday's party.

The participants lived up to their billing as unusually sprightly. One woman in her 80s, wearing designer sunglasses, tried to recruit people for an after party at Gibson's, a Chicago steakhouse and bar. At one point, Mr. Karczmar, a self-proclaimed socialist, pumped his fist in the air, punctuating a joking attempt to unionize fellow participants.

Don Goldsmith, 84, an avid tennis player and golfer, said he volunteered to contribute to research he hopes might eventually eradicate Alzheimer's. He said his friends needle him over his status. "They say, 'If you're a SuperAger, how come you can't hit a golf ball?' "

By 6 p.m., the party had wound down. Mr. Goldsmith was gone, having rushed out an hour earlier for another shindig. "I always said I believe in the two-party system," he bellowed on his way to the door.

For some unmarried SuperAgers, the event had offered hopes of finding a date among like-minded peers. But even at this age, asking somebody out can be complicated. "I saw one fellow and said, 'Hello,' " said Shirley Lowry Haas, 89, as crews cleaned up and folded chairs. With a wry smile, she added, "All of a sudden, his wife says, 'He belongs to me.' So I kept quiet after that."