My great grandfather was born in 1899. He never fought in any war. His older brothers went off to WWI. I always assumed my great grandfather was too young to go to war in WWI. But apparently not.
He would have been 18 in 1917 when the US declared war. There could have been a lot of reasons he didn’t go while his older brothers did. Back then, they used to reject you if you had ‘flat feet’. Not every man of military age served in that or any other war. Thankfully, we have never been in that position.
It was the same with my grandfather. Born in 1899, he would have been old enough to enlist by 1917 but didn't. Even my father isn't too clear on the reasons. Granddad did have a defense-related job -- he was a tool & diemaker for an automotive company so he probably had a draft deferment. Of course, he would have been too old for WW2 (though it wasn't unheard of to find 40 year old infantrymen).
My family was of German extraction, so perhaps that had something to do with it?