My first thought was along #2's lines- the young ones don't read enough to justify having an eReader. Of all the students I see in a day, I can only think of two who are avid readers, and they both read on their laptops.
I think another difference occurs in reading matter. While older readers choose mainstream books to read, the younger generation reads things that are not yet available for eReaders; graphic novels, peer work on sites like fanfiction.net, etc.
I also suspect that eReaders lack appeal to younger readers because they are not interactive enough.