Bob Dylan turned 80 the other week. Does that seem to consign not just the vanished twentieth century but the long stretch of modernity we identify with to the filing cabinet of history? Well, maybe not. Before the Sixties, everyone thought that rock music, pop music– whether it was Elvis or Little Richard – was a pastime for teenagers but every-one would eventually put away the things of childhood and invest in jazz or the real blues of a Robert Johnson, in folk, or indeed in classical music.