Awhile ago I read about a guy that was developing Alzheimers. He decided to kill himself before it got bad. He made a website and everything, detailing his decision. It's probably online if you look for it.
But I think I agree with him. If I was pretty certain I was going to end up unable to care for myself and wouldn't be able to change that, I think I would want to kill myself before that happened as well. I wonder if it would be humane to assist someone in suicide if they can make a sound case for it?
There might be thread on this, I don't know.