Updated: Feb 5, 2019
In case you haven't already, look over your tree and make a list of all the men born between about 1873 and 1900 and were still living in 1918 and then head over to FamilySearch.org where you can enter those names and search to find their draft registration cards for World War I for free. (Click here for more details).
I found draft registrations for three of my four great-grandfathers, Albert Leonard, Percy Henry Daniels, and William Henry Dickinson (shown below). The other great-grandfather, John Reese, was born in 1910 and was too young.