What makes you so sure?

Watch series 1 again now - bearing in mind that all the characters were adults already when the show began - and you'll see that they have definitely aged. People don't age all that much from 50 to 60 or from 20 to 30. But take another look at Cora and the Dowager - they've visibly aged. So have Mary and Edith. …

Wrong. The death rate of officers, who were drawn from the upper classes, was - at least until 1916 - much higher than that of enlisted men. They were easily recognizable by their uniforms and the Germans picked them off first. Starting in 1916, officers didn't wear distinctive uniforms anymore.