I stopped teaching about the time I was losing touch with what I could reasonably expect my students to know and not know with respect to relatively recent history and popular culture. I have a sense that things are actually a bit different now than when I was an undergrad, in that there is a somewhat sharper break from the past for students of that age. I'm not talking about them being less educated, I'm talking about the basic culture that they become aware of through fairly automatic processes.
But, yes, it is a challenege for teachers.
