K.M.
Let the world teach her this lesson is the road I have taken with my son. I think humor is something you both learn (like the art of joke telling) and something you are born with ... you can try to help her understand why it's funny and why it's not funny, but I do not think there is an age when this magically happens.
I can laugh at a joke, I am funny in the things that I do or ways that I tell a story that is funny (or so I am told) but I can not tell a joke - at all - ever.