JJV's post below* coincided more or less with my perusal of this overview of the world's known surviving veterans of World War I (h/t Lawyers, Guns and Money). I found it amazing that the last surviving veteran of the Russian-Polish War managed to make it through everything else that came his way over subsequent decades.
Like EMM my paternal grandfather went "over there" (after first coming here from Ireland) and also got gassed. Luck o' the Irish, I suppose.
*I, too, hanker for a world without penicillin. Sorry, couldn't resist.