What I went through was vanishingly small compared to my grandmother and father. It cemented in my dad a resolve to self-euthanize at the first sign of a progressive mental degradation. Of course, the first sign is "too early" to be optimal as well, but no one wants to wait until it's too late. In a strange twist of fate, he experienced this fall a temporary but rapid decline in function due to a treatable (and subsequently treated) physical condition that manifested itself over a 10 day period. Now recovered, I'm sure he's glad he didn't off himself.

I don't have any easy answer, even having had these experiences. There's little question my grandfather would not have wanted to live as he did the last 18 months of his life, but still epsilon chance he prefered whatever his life was to nothingness and even if not, was 17 or 19 a "more right" answer? How do you distinguish "routine" "what did I come downstairs for again?!" from more serious progressive degenerative diseases? Who knows...