@Viss

For most scholarly work, 1994 is indeed a bit old. I dimly recall being told to try to keep my citations to within the last ten years.

The point is to make sure your arguments are backed by recent scholarship. But it's not by any means a hard and fast rule. There are many reasons one might cite an older source and, if you use them, you need to cite them like anything else.

The professor who posted this is of course well aware of all of this but I do occasionally hear about odd restrictions on citations. I'm definitely not a fan of such restrictions: We need to be encouraging more literature searching, not less.