How old is the modern era? When exactly did olden times turn into modern times? Are the Victorians modern? Of course not. Is H.G. Wells? No, not quite. Is Charlie Chaplin? No - though Buster Keaton may be. My guess is the First World War marks the point after which you no longer find yourself thinking autre temps, autre moeurs. Pre-war people (that is to say, people whose characters were formed before the war) and post-war people (those whose characters were former during or after it) are two different species of human being. It's as if, during those four years, human evolution put on a spurt.
And what exactly is the difference between the two species? I think it's this, that modern people find it difficult to take themselves entirely seriously.