4 in 10 Americans at risk for dementia after age 55, study suggests

New cases of dementia in the United States are projected to double in the next three decades, a new study suggests.

The study, published this week in the journal Nature Medicine, looked at more than 15,000 people and estimated the lifetime risk of dementia from ages 55 to 95.

The team -- including researchers from Johns Hopkins University, Mayo Clinic and New York University -- projected new U.S. dementia cases would double from more than 500,000 in 2020 to approximately 1 million by 2060. The authors said this increase is directly tied to the aging of the U.S. population.

The study also showed that the risk of developing dementia after age 55 is 42%, more than double the risk seen by older studies. After age 75, the lifetime risk increases to more than 50%, according to the study.

"Our study results forecast a dramatic rise in the burden from dementia in the United States over the coming decades, with one in two Americans expected to experience cognitive difficulties after age 55," Dr. Josef Coresh, a study senior investigator, epidemiologist and founding director of the Optimal Aging Institute at NYU Langone, said in a press release.

Greater risks were seen in women, Black Americans and people who carry the APOE4 gene, a gene that primarily increases the risk of Alzheimer's, according to the study.

Researchers also found that white Americans are expected to see nearly double the number of dementia cases, while cases among Black Americans could triple.

"These results highlight the urgent need for policies that enhance healthy aging, with a focus on health equity," the authors wrote in the study. "Understanding the lifetime risk of dementia can inform public health planning and improve patient engagement in prevention."

Dementia is an umbrella term to describe a decline in cognitive abilities, including loss of memory and trouble concentrating, that can impact daily life and ones ability to function independently.

There are a number of diseases that are grouped under dementia including Alzheimer's disease, Huntington's disease, Lewy body dementia, frontotemporal dementia and vascular dementia.

An estimated 10% of the population aged 65 and older in the U.S. has dementia, according to the Centers for Disease Control and Prevention.

Although aging is a risk factor for dementia, there are other genetic factors that increase the risk, as well as high rates of diabetes and hypertension, obesity, unhealthy diet, lack of exercise and poor mental health, the study authors said. The study also used more comprehensive methods to identify dementia diagnoses, which could have increased the projection of new cases. 

"The pending population boom in dementia cases poses significant challenges for health policymakers in particular, who must refocus their efforts on strategies to minimize the severity of dementia cases, as well as plans to provide more health care services for those with dementia," Coresh said.

ABC News' Dr. Dana Lin contributed to this report.