The finding follows a decade spent tracking high blood pressure and dementia among 625 men and women aged 90 and up.
Those with the highest blood pressure levels were the least likely to have dementia, the researchers found. But that doesn't mean older people shouldn't try to control elevated blood pressure, they said.
"On the basis of this work we are absolutely not recommending that high blood pressure not be treated among the elderly," said study co-author Maria Corrada, an associate adjunct professor in the department of neurology at the University of California, Irvine.
"What we are saying is that from observing a group of very old people we now have some evidence that developing high blood pressure at a late age may be helpful in terms of maintaining intact thinking abilities," she said.
The reason behind this association isn't clear, Corrada said. "It could be that high blood pressure improves the blood flow to their brain... But we don't know. It will certainly require more study to better understand the mechanism behind this."
The study didn't prove that high blood pressure leads to improved mental acuity among the very old, it just found a link between the two.
Corrada and her colleagues are scheduled to discuss their findings this week in Copenhagen, Denmark at an international meeting of the Alzheimer's Association. Research presented at meetings is considered preliminary until published in a peer-reviewed medical journal.
For the study, the study authors focused on men and women aged 90 to 103 -- the "oldest old."
Almost seven in 10 participants were women, and at the start of the study none had dementia. All underwent an initial battery of mental health and neurological testing, as well as blood pressure assessments.
Then, for up to 10 years, all participants were re-evaluated every six months.
Overall, 259 were diagnosed with dementia. And nearly three-quarters took some form of medication to control high blood pressure, which is linked to heart disease and stroke.
The research team found that those who had developed high blood pressure during their 80s faced a lower risk of developing dementia than those with normal blood pressure. And participants first diagnosed with high blood pressure during their 90s had an even lower dementia risk.
Even those whose blood pressure was slightly elevated -- called "pre-hypertension" -- had a lower risk for dementia than those with normal blood pressure, the study found.
The absolute lowest dementia risk was seen among those whose high blood pressure was the most advanced. The association held whether or not people took medication for their high blood pressure.
This led the research team to conclude that high blood pressure seems to be associated with a lower risk for dementia among the extreme elderly. Moreover, the worse a person's blood pressure status is, the better their thinking capacities, the researchers said.
"Now it should be said that people who survive to age 90 and above are, by definition, very different than people who survive to their 70s or 80s," said Corrada. "Obviously, they didn't have stroke or heart disease that took their lives. Or maybe they didn't have it at all. And more specifically, these may be people who have developed high blood pressure now, but did not struggle with it throughout their life."
Catherine Roe, an assistant professor of neurology at Washington University School of Medicine in St. Louis, said the findings seem to "fit with what we know about high blood pressure."
"High blood pressure during middle-age is a risk factor for later development of dementia," Roe said. "That's pretty clear from past research. But the study results have been mixed on whether high blood pressure for people in their 60s and 70s is related to cognition problems or not." Cognition refers to thinking and memory.
It appears that high blood pressure becomes less of a risk factor for mental decline in old age, Roe added. However, "high blood pressure is probably still bad for a lot of other reasons," she said.
Perhaps people who live to very advanced ages have a kind of "super health" that helps them withstand the impact of high blood pressure, she suggested.
For more information on Alzheimer's, visit the U.S. National Institute on Aging.