Just spitballing but maybe the same reason why we generally see shingles in older people. Weakened immune system in older age leads to the reactivation of the chickenpox causing virus.

Braaks hypothesis actually has some evidence behind it, (one hypothesis that says sporadic Parkinson’s can be caused first through the olfactory or gastrointestinal system infections). Some flaws too, but interesting to read about.

Also it has been known for a while now that several forms of dementia are preceded sometimes a decade earlier by psychiatric manifestations, almost like the prodromal stage to the upcoming shitstorm. Basically manifests differently in the more middle aged, or right before geriatric population.

An interesting example is that true REM sleep behavior disorder has a 70% chance to progress into a neurodegenerative condition like Parkinson’s Disease. By true I mean diagnosed by experienced neurologist or sleep specialist and ruled out all forms of apnea, antidepressant use, stimulant use, poor sleep hygiene, etc.

Lastly, schizophrenia in the past used to be called dementia praecox before we understood what was going on. They are finding now that some forms of dementia (such as behavioral variant of frontotemporal dementia) actually mimic Alzheimer’s but have important earlier manifestations. Really cool stuff down the pipeline, and maybe decades down the road we will have much better screening and treatment measures.