I think grumpy old people are funny because they have such short memories. My brother-in-law (not all that old at 60) moans about the rudness and cockiness of young people, completely forgetting that he himself was a total thug, got sent to borstal and did not become a model citizen and reformed character until he was at least 45!!
My work with young people has taught me that they can be very grumpy. Give them more than half an A4 page of font 16 to read and they will huff, puff, tut, throw their legs out and sink into the chair and whine. tell them they have to work the whole two hours without a break and they start all over again. Tell them off for ariving late without pens and the proper equipment and they will whine "It's not my fault!! I got up late!!"