Roger Ebert linked to Bill Mesce's post "The 'Gray Ones' Fade to Black", about a subject I've touched on a lot: people who came of age in the '60s, '70s and '80s did so at a time when television was filled with old movies as cheap filler programming, and so old black-and-white movies grew up unusually familiar with movies (and TV shows and animated films) made before they were born. Today, it's much easier to get programming that suits your particular tastes at any time, so it's much less likely that someone today will grow up watching Humphrey Bogart or Abbott and Costello.
Mesce goes into a lot more specifics about how television made old movies into a part of the cultural conversation for Baby Boomers and even successive generations. (There were lots of late movies when I was a kid in the '80s, and it was as late-night filler that I first saw everything from MGM musicals to Ingmar Bergman movies.) And he explains how and why TV stations, and then cable networks, mostly gave up the black-and-whites. It's worth reading.
Now, having said that it's worth reading, I feel a little uncomfortable with the implication that today's young people have unusually little knowledge of old movies. Probably the truth is the other way around: the '60s through the '80s were the exception. And even that era had huge gaps in its cultural knowledge. Silent movies didn't turn up on TV all that often, and as one of Mesce's commenters pointed out, today there's much more interest in Pre-Code movies than there was before. (The Pre-Codes inspired a huge amount of interest in part because a lot of them didn't play on TV very often.)
But anyway, movie studios have always known that young people -- and old people too -- would prefer something new. So have TV networks, publishing companies, popular song publishers. (When old stuff eclipses the popularity of the new stuff, that's a sign that the form is dying, like opera has been mostly dead for the last 50 years or more.) And there has always been an assumption that audiences won't get references to old stuff in the same form: one of the reasons Sunset Blvd. was such an unusual film at the time was that it was filled with references to the past of movies -- movies of less than 25 years ago! -- and the theme of the film is that the public and industry alike have forgotten all about people like Swanson, Stroheim and Keaton.
Television, art-house revivals (not to mention the arrival of old American movies in bunches overseas) and the early '70s nostalgia boom helped to change that, but it wasn't a normal state of affairs. Concentrating on new stuff, in a recognizably contemporary style, is the normal way. A contemporary style can be assimilated naturally. Experiencing older styles is like work. And that work is, in a way, harder for "entertainments" than for works that are supposed to be difficult. Last Year At Marienbad is recognizably an early '60s movie in style, and the modern viewer has to adjust to that, but it was always intended to be something the viewer had to work at. But a great American commercial film was supposed to be easily accessible, and as time goes by and styles change, it's no longer easily accessible. So the point of a great American commercial picture -- that it is both a simple entertainment and something with resonance beyond that -- is lost. For a lot of viewers it no longer works on the simple entertainment level, and until it works on that level, it won't yield deeper meanings either. A John Ford Western doesn't start to seem profound unless it first works as a conventional Western.
So I can't criticize people for not growing up as old movie buffs; I don't think that's normal. I think it helps to adjust accordingly, don't assume film students know who Humphrey Bogart is, explain who he is and why they should care, just as we would explain who any old dead guy was and why he was great. Explain the grammar of old movies and other things they may not expect. For example, and to go back to John Ford for a minute, his visual style makes more sense if it's explained the way he himself explained it to Steven Spielberg -- he was a painter who tried to set up shots with a painter's eye. People can learn about older ways of doing things, they just won't grow up being used to them, that's all. That was abnormal.