Forgive the ignorant question: if the chance spikes to ~50% by one's 80s, is Alzheimer's really a disease per se, or is it just a natural progression of the brain degrading due to old age?
I mean, some people try to label the physical aging process itself as a "disease", as if it needs to be "cured" via discovering a path to immortality. Does Alzheimer's fit into that line of thinking, or is it truly a "real" disease.
Short Answer: Yes, both forms of Alzheimers are a "disease". Early Onset Alzheimers may strike people as early as their 30s, though it may not be diagnosed for a decade or more. This type of the disease has been definitively linked to gene mutations. At the other end of the spectrum is what more primitive people once called "senility" or "old age" giving rise to your question --that form of Alzheimers strikes about half of people who are 85 or older (though they will likely have started having symptoms in their 60s). The (very) latest theories on this form of the disease is that it may be linked to auto-immune responses related to various forms of Human Herpes viruses (for example VZV, HSV1, etc.) and may even spread to brain surgeons during operations. Disclaimer: I am not a medical doctor.
Bill Gates has started funding research. Hopefully, we can address the disease in some form within a decade or two.
https://www.gatesnotes.com/Health/Digging-Deep-Into-Alzheime...
https://www.gatesnotes.com/Health/How-you-can-help-fight-Alz...