Dementia Risk After 90: New Study Shows Why We're Never "In the Clear"

It is a common and comforting assumption: if you or your loved ones can make it to 90 years old with a sharp mind, you have somehow beaten the odds. We tend to think that after a certain milestone, the risk of cognitive decline plateaus, and we can finally breathe a sigh of relief.

But is that actually true?

Until recently, scientists had very little data on what happens to our brains in extreme old age. We have historically focused almost all of our research on early-to-midlife risk windows or on those aged 65 to 85. But as global healthcare improves, people are living longer than ever before. In fact, demographers estimate that by the year 2100, there will be roughly 230 million people aged 90 or older living on Earth. Understanding how the brain ages in this rapidly growing group—the "oldest-old"—is no longer a niche research topic; it is an urgent public health necessity.

A groundbreaking study published in The Lancet Healthy Longevity by researchers at UC Davis Health and Kaiser Permanente is finally shining a light on this mystery. The findings from this project, known as the LifeAfter90 study, deliver a powerful wake-up call: reaching age 90 without cognitive decline does not plateau or lower future risk. The biological vulnerability to Alzheimer’s pathology remains elevated and continuous, even deep into our tenth decade of life.

What is the "LifeAfter90" Study?

Launched in 2018, the LifeAfter90 study is a major scientific effort led by Dr. Rachel Whitmer, a professor of public health sciences and neurology and the chief of epidemiology at UC Davis Health.

The study is unique for two main reasons

• Healthy Enrollment: Every single participant enrolled in the study was at least 90 years old and had zero signs of dementia at the start. Researchers then followed them closely, performing cognitive examinations every six months.

• Unprecedented Diversity: Unlike many historical clinical trials that featured predominantly white, affluent cohorts, this analysis looked at a highly diverse group of over 800 individuals (with a median age of 92).

Because these participants are long-term members of Kaiser Permanente, researchers didn't just study them in their 90s; they also had access to decades of their medical records, in some cases dating all the way back to the 1960s. This rich, long-term data allowed them to trace how health habits in midlife affect the brain decades later.

The Core Findings: High-Risk Demographics and Disparities

The study's most striking revelation is that the demographic disparities in dementia risk we see in younger seniors don't disappear after age 90. In fact, they remain just as pronounced.

1. The Significant Gender Gap

In younger populations, women generally show higher rates of dementia than men, but researchers didn't know if this trend held true for those who survived past 90. The LifeAfter90 study answered this question with a resounding yes: women aged 90 and older have roughly twice (2x) the risk of developing dementia compared to men of the same age.

2. Racial and Ethnic Disparities Persist

"It is striking that the racial and ethnic disparities in dementia risk observed in younger adults continue into the tenth decade of life," noted Dr. Hilary Colbeth, a UC Davis postdoctoral scholar and the lead author of the study.

The study found that Black and Hispanic participants faced significantly higher rates of dementia than their white and Asian peers. Most notably, Black participants had a 75% higher risk of developing dementia than Asian participants. This points to systemic health disparities and life experiences that continue to impact brain health even after nearly a century of life.

Decoding the Genetics of Aging: The Role of the APOE Gene

Our genetics play a powerful role in how our brains handle aging. The researchers specifically looked at a gene called APOE, which is responsible for how our bodies transport cholesterol and is strongly linked to Alzheimer's disease. Everyone inherits two copies of the APOE gene (one from each parent), and it comes in three main variants: APOE2, APOE3, and APOE4.

The Protective Shield: APOE2

For those carrying the APOE2 variant—which is historically linked to a lower risk of cognitive decline—the protection remained incredibly strong. Participants in their 90s who carried this variant enjoyed a 60% lower risk of dementia than those without it.

The Double-Edged Sword: APOE4

The APOE4 variant is the most well-known genetic risk factor for Alzheimer's. Interestingly, when looking at the LifeAfter90 study population as a whole, APOE4 did not significantly raise the overall baseline of dementia risk. However, when the researchers broke the data down by demographic subgroups, a much more complex and concerning pattern emerged:

• Among Men: Carrying the APOE4 variant was associated with a higher risk of dementia.

• Among Black Participants: Carrying the APOE4 variant approximately doubled the risk of developing dementia.

This underscores the reality that genetic risk factors do not act uniformly; they interact with our sex, race, and ethnicity in ways that science is only beginning to untangle.

The Mystery of Cognitive Resilience

While many of the findings highlight ongoing risks, the LifeAfter90 study also revealed a fascinating and hopeful mystery.

Some participants entered their 90s with perfectly healthy, sharp minds, despite having well-documented risk factors—such as high blood pressure (hypertension) and high cholesterol—during their 60s and 70s. By all traditional medical logic, these individuals should have been at high risk for cognitive decline. Instead, they showed incredible resilience.

These "resilient brains" are now a major focus of scientific curiosity. If researchers can understand what biological shields or lifestyle factors allowed these individuals to resist brain damage despite their cardiovascular risk factors, they could unlock new therapies to protect the rest of us.

What Does This Mean for the Future of Healthcare?

The takeaway from the LifeAfter90 study is clear: we cannot afford to stop focusing on brain health just because someone has reached an advanced age. Reaching extreme old age does not remove biological vulnerability, underscoring the vital need for preventative interventions that extend late into life rather than focusing solely on early-to-midlife windows.

"We can't just assume that someone from a high-risk group makes it to 90 without dementia and they're in the clear," warns Dr. Rachel Whitmer. "We need to talk about risk reduction for everyone."

For doctors and clinicians, this means continuing to monitor, support, and discuss cognitive health with nonagenarians. For family caregivers, it means remaining vigilant and advocating for the older adults in their lives. And for policymakers, it means funding preventative brain-health interventions that span a person's entire lifetime—from childhood education and environmental protection (like those investigated in UC Davis's concurrent Neighborhoods Study) all the way into extreme old age.

While we may never be entirely "out of the woods" when it comes to dementia, studies like LifeAfter90 provide us with the map we need to navigate the path forward, ensuring that our longer lives are also healthier, sharper, and more vibrant.

Disclaimer: This blog post is based on peer-reviewed research published by UC Davis Health and Kaiser Permanente in The Lancet Healthy Longevity (August 2026). Always consult a healthcare professional for personalized medical advice.