Note that Hileman didn't join the Army until 1919 and actually wasn't involved in what we understand as WW I at all.
Instead, he served in the expeditionary force that was sent to Siberia after Armistice Day -- in an attempt to stabilize Russia in the wake of defeat and post-war revolution.
I imagine that most of the other hundred-or-so survivors of WW I were comparative youths who also saw service in Russia, post-Armistice.
My grandfather died in 1993. He was a Missouri farm boy who volunteered for service in WWI by lying about his age. He got as far as Goat Island before the war was over.
He subsequently went to the Naval Academy and then married above his station. The night of Pearl Harbor, he volunteered. Since he was a fairly old man at the time (approaching 40), the Navy had him teaching ROTC. He wanted to do something more and volunteered for combat.
He was on the Bunker Hill as damage control officer during the battle of Okinawa. He was the oldest man on the ship. If you have ever seen Victory at Sea, that is the Bunker Hill with the massive plume of smoke coming off the flight deck.
He put on a breather mask, went below and formed a human chain to bring men out. And then worked to keep the ship afloat until they reached Bremerton.
He would be 104 now if he were still alive. I miss him very much.