TLDR: Some studies on what causes Alzheimer’s disease are examining the link between the early presence of one of the hallmark chemical brain chemicals (plaques—or clumped buildup—of the protein beta-amyloid) and later development of the disease.
People 18 to 65 who experience high stress, anxiety and depression seem to have greater amounts of this marker protein, and statistically greater risk of developing Alzheimer’s in later years.
Does untreated mental illness tend to cause higher rates of Alzheimer’s? Does impending Alzheimer’s (accompanied by increased beta-amyloid) cause mental health issues? Can early intervention lessen the risk of developing Alzheimer's later? Is the connection mere coincidence?
The cause-and-effect connection is still unclear. (Caveat: No, if you’re depressed, this does not mean you’re headed for dementia-land.)
I can attest to my own experience with my partner, who was officially diagnosed with early-onset Alzheimer’s at age 61. She had about a decade’s worth of whack-a-doodle mental health issues prior to the diagnosis. None of the conventional therapies (drugs, counseling) seemed to work.
We know she had a hereditary predisposition (her father and 2 aunts had some form of dementia). We know she had a childhood that caused anxiety and depression. Perhaps it was a snowball effect—the biochemistry of mental health issues poking and prodding the genetic component into action. Or perhaps the mental health and Alzheimer's symptoms merely overlapped.
Some further reading on current research into links among beta-amyloid, mental illness and Alzheimer’s:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8534102/