You say: "and still they don't get the message!". While of course I agree that it was a stupid 'joke' and the remark should not have been made, he was a sixteen year old kid, probably on his first trip without his parents and therefore the first time he had personally been asked the security questions. While those of us who work in the industry all the time are all too aware of "the message" not to joke about this subject, it isn't widely known among the very occasional and first-time pax, as evidenced by the number of times this happens. Exactly where can such pax be expected to get "the messsage", especially a kid?
I think that some form of 'caution' should preface the security questions, either printed on or with the ticket, or on a notice on the check-in desk, with the notice being brought to the passenger's attention before the questions are asked.
Incidentally, and with reference to this incident, I've always wondered what would happen if the answer to the question: "Did you pack the bag yourself?" was given as: "No, my Mum did".