My parents are in their 90's and both dumb as bricks. As they have always been.

Here is a joke I have told no one about them:

You see, unlike my Father, my Mother isn't low class. She's refined... As where he frequents bars that offer gambling, she being of an upper class, frequents casinos that offer you free drinks!

I've been working on that one for about a decade now. Let me know if it lands.