While most of the 5.3 million Americans who are living with Alzheimer’s disease or other forms of dementia are over 65, some 200,000 are younger than 65 and develop serious memory and thinking problems far earlier in life than expected.

“Complaints about brain fog in young patients are very common and are mostly benign,” Dr.

The Dutch study found that overall, Alzheimer’s disease was the most common cause of young-onset dementia.

But when symptoms developed before age 50, early-onset Alzheimer’s was a less likely explanation than two other causes: vascular dementia and frontotemporal dementia.

In frontotemporal dementia, portions of the brain that lie behind the forehead and ears shrink, resulting in dramatic personality changes, socially inappropriate or impulsive behavior and emotional indifference.

Lewy body disease is another cause of dementia in younger adults.

Alzheimer’s disease remains the most common cause of dementia in younger as well as older adults.

There is an inherited form of Alzheimer’s that typically arises at younger ages, but those cases account for fewer than 10 percent of young-onset disease.

People with Alzheimer’s typically have a buildup of abnormal substances — tau and beta-amyloid proteins — in the brain.

Brain scans may show a loss of brain cells and an impaired ability to metabolize glucose that is indicative of degenerative brain disease.

Probably the most publicized factor known to increase the risk of early dementia is repeated head injuries like those experienced by professional boxers, football and soccer players, and sometimes by military veterans.

Knopman said he’s less concerned with elementary school children playing such sports; the risk of developing dementia at a young age from repeated head trauma is much greater among those who played Division 1 football or became professional boxers.

Body-wide inflammation linked to diabetes and heart disease can cause brain changes that promote dementia.

And a glucose PET scan can uncover abnormal patterns of sugar uptake in various parts of the brain that can help distinguish between Alzheimer’s disease, Lewy body dementia and frontotemporal dementia