@Laotionne - I can relate to the character from the book you mentioned who had early onset dementia. Anybody who has ever been around a family member or friend with this condition knows how difficult the disease is on the person who has it, and on everybody around her.
As much as I wouldn't want to put my family through that ordeal, I couldn't take my own life because you never know when medical science is going to find a cure or treatment that really works. Also, who knows what is actually going on in a person's head when she has dementia. Maybe there is something productive happening, but only that person is aware of what's going on.