Reaching 90 without dementia may feel like clearing the final hurdle. But the risk does not necessarily end there.
Dementia risk does not disappear after age 90. Even among people who reach their 10th decade without cognitive symptoms, the likelihood of developing the condition can vary substantially by sex, race, ethnicity, and genetics, according to new research that challenges long-held assumptions about brain aging.
Understanding those risks is becoming more urgent. The number of people age 90 and older worldwide could reach about 230 million by 2100, yet this age group remains poorly represented in brain aging research. Most of what scientists know about dementia comes from studies of younger older adults, often in populations that lack racial and ethnic diversity.
LifeAfter90 Tracks Dementia Risk
The LifeAfter90 study was designed to address that blind spot. Since 2018, researchers at UC Davis Health and Kaiser Permanente have followed a diverse group of Kaiser Permanente members who enrolled at age 90 or older before showing signs of dementia. Participants receive cognitive evaluations every six months, allowing the team to identify new cases and examine how risk changes at exceptionally advanced ages.
The latest analysis, published in The Lancet Healthy Longevity, included medical records from more than 800 participants with a median age of 92. Because many had received Kaiser Permanente care for decades, the researchers could also draw on health histories that sometimes extended back to the 1960s.
That long record is especially valuable because dementia develops over many years. Conditions and experiences in a person’s 60s or 70s may influence cognitive health decades later, even when symptoms do not appear until after age 90.
“We know from other studies, done in people 65 and older, that there are differences in dementia rates, and women tend to have higher risk, but nobody knew if that was true after 90,” said Rachel Whitmer, a UC Davis Health professor of public health sciences and neurology, chief of epidemiology, and senior author of the study. “We need to understand who is most affected by dementia after 90 and how the main Alzheimer’s risk gene (APOE) factors in.”
Dementia Disparities Persist After 90
Women in the study were twice as likely as men to develop dementia. The pattern resembles findings in younger populations, showing that the difference does not disappear simply because someone has reached an unusually advanced age.
The study also uncovered substantial racial and ethnic disparities. Black participants had a 75% higher risk of dementia than Asian participants. Black and Hispanic participants experienced significantly higher dementia incidence than white and Asian participants.
“It is striking that the racial and ethnic disparities in dementia risk observed in younger adults continue into the tenth decade of life,” said Hilary Colbeth, a UC Davis postdoctoral scholar in public health sciences and first author of the paper. “Specifically, Black and Hispanic participants had significantly higher dementia incidence rates than white and Asian participants.”
APOE Genes Shape Alzheimer’s Risk
The researchers also examined APOE, a gene involved in transporting fats through the bloodstream and a major focus of Alzheimer’s research.
Its variants do not guarantee that someone will or will not develop dementia. Instead, they alter probability. APOE2 is generally associated with lower Alzheimer’s risk, while APOE4 is the strongest common genetic risk factor for late-onset Alzheimer’s disease.
In the LifeAfter90 group, APOE2 remained strongly protective. Carriers had a 60% lower risk of developing dementia, suggesting that the variant can continue to matter even at exceptionally advanced ages.
APOE4 Risk Varies Across Groups
APOE4 produced a more complicated result. It did not significantly increase dementia incidence when researchers considered the entire study population together. Once the data were separated by sex and ethnicity, however, important differences emerged. APOE4 was associated with greater risk among men and nearly doubled the risk among Black participants.
“We saw evidence that APOE4 impacts males and females differently after age 90,” Colbeth said. “This has prompted us to look more closely at how APOE genotypes impact mortality among those with and without dementia by age 90.”
The finding may help explain why genetic risk estimates developed from one population do not always apply equally to everyone. It also highlights the value of including diverse participants in aging research rather than treating results from largely white study populations as universal.
Why Some Brains Remain Resilient
The study raised another question that its current results cannot answer: Why do some people remain cognitively healthy despite carrying recognized risk factors?
Several participants had experienced high blood pressure, high cholesterol, or other conditions associated with dementia during their 60s and 70s, yet they reached their 90s without cognitive impairment. Some APOE4 carriers also remained cognitively healthy.
Researchers now want to identify what protected those individuals. The explanation could involve biology, medical care, behavior, social conditions, or a combination of factors. Discovering those sources of resilience could eventually reveal ways to help more people preserve cognitive function.
“Doctors need to know that certain groups are at higher or lower risk,” Whitmer said. “We can’t just assume that someone from a high-risk group makes it to 90 without dementia and they’re in the clear. We need to talk about risk reduction for everyone.”
Reference: “Incidence of dementia after age 90 years and association with APOE genotype, race, and sex in the USA: the LifeAfter90 prospective cohort study” by Hilary L Colbeth, Maria M Corrada, Dan Mungas, Paola Gilsanz, Kristen M George, Reham Gaied, Claudia H Kawas, Charles DeCarli and Rachel A Whitmer, 4 August 2026, The Lancet Healthy Longevity.
DOI: 10.1016/j.lanhl.2026.100882
This research was supported by the National Institute on Aging of the National Institutes of Health (R01AG056519 and P30AG072972).
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