Ethical Suicide?

I've been thinking about my Grandpa alot lately. He used to be such a fun person to be around. I idolized him as a kid and a young man. He was wonderful with people. He had such a fantastic sense of humor that made him fun to be around.

Now he has gotten old, lost the use of his legs, and his bowels. I feel bad for him. My mom told me a long time ago about a humorous comment he made a long time ago. Something like "When I get so old that I'm shitting my pants, that's when I hope I go." When you get to that point I can imagine that can be a struggle with your self esteem. My grandpa has drawn inward so much now that he hardly speaks.

I tend to put myself in his shoes. What would I do in this situation? Or in the situations leading to this part of his life? I have rather contrary views on a few things than my wife and parents. One of which is how I feel my old age and end of life should be handled. Is it ethical for someone to decide for himself, when his life has been completely used up?

I read this document that expanded some of my thoughts. This one took more of a statistical and scientific approach. When a horse breaks a leg, often they're shot, or when a dog gets so old that he can't go up stairs or get up very well, he is euthanized. I know humans aren't animals, but why all of a sudden is a human supposed to live his life until it ends completely naturally, even at the expense of himself and others. I would argue it's an individual decision and shouldn't be a blanket thought that its wrong for everyone in any situation.