I would also like to add that Alzheimers is a terrible disease, and is not funny. However, most of us lose the elasticity of our memory as we age. And that, while frustrating, can be funny. Every joke about aging is not a joke at the expense of those with Alzheimers. I had to read the joke twice before I noticed the last line. It's funny without it.
To anyone dealing with this terrible disease, jokes like this may be hurtful. But for all of us who are merely aging with bizarre side effects, it gives us a chance to laugh at ourselves. I hope no one is offended or hurt....