As I get older, and as I am invited to speak in a wider range of locations, I am becoming increasingly aware that many people in my adudiences have a very different set of shared idioms and examples. Indeed, I feel that this is such a big issue that I plan to write a manual about it - when I get the time!
I am calling this problem the Divided Knowledge problem. The problem is already well known to academics, and has an academic name, i.e. restricted code. But the use of the phrase restricted code is itself an example of Divided Knowledge.
A really good example of the Divided Knowledge problem happened to me the other day when I was advocating that Burma/Myanamar should be isolated in the way that Rhodesia was isolated. Luckily, I happened to notice the number of blank expressions when I used the example of Rhodesia. I did a quick check, and not one person in the room under 30 was aware of Rhodesia. For people over 40 it was a very clear indication of what I meant.
The illegal state of Rhodesia was created in 1965 by its UDI (Unilateral Declaration of Independence) as an attempt to maintain minority white control, in the country that is Zimbabwe. The white minority Government was finally forced into 'one person one vote' in 1979, and in 1980 a majority Government was elected and the country renamed Zimbabwe.
Divided knowledge based on age can be really difficult to spot and avoid. We expect to see it when dealing with areas of speciality, physicists have a different restricted code to the art historians, and we expect to see it. Young people and older people have different restricted codes based on youth culture, and we expext to see it. But there are clear generational effects in knowledge about major events. Some like 9/11 are probably going to stay in the shared knowledge for a long time, but others, such as Rhodesia are a major event for some age groups, and are meaningless to people just a few years younger.
Increasingly, I am requesting the people who kindly proof read my papers and presentations, and the people who suffer the agony of my rehearsals being inflicted on them, to flag up anything which seems odd - because the chances are that it may not be odd, simply an example of Divided Knowledge.
BTW, most people under 20 can't name all four of the Beatles, how shocking is that?