"During World War I there were three registrations. The first, on June 5, 1917, was for all men between the ages of 21 and 31. The second, on June 5, 1918, registered those who attained age 21 after June 5, 1917. (A supplemental registration was held on August 24, 1918, for those becoming 21 years old after June 5, 1918. This was included in the second registration.) The third registration was held on September 12, 1918, for men age 18 through 45."
So your doughboy could possibly be in the 1880, 1890 (what remains of them), 1900, 1910, 1920, 1930 and 1940 censuses, all of which are released to the public for genealogical research. Of course they could have been also enumerated in more that those, but the 1950 census has yet to be released, along with all future censuses. (I grieve that our progenitors will not have the robust censuses of the past as they lack the wonderful data of past ones).
Where to find the census records: FamilySearch.org has all of them from 1790 through 1940 for your use to search for free. I have linked to one of their blog articles which tutors the reader about how to use the records. As the blog article points out:
"Census details vary from year to year, with some of the more recent ones offering the most details. Since 1880, censuses can often help you reconstruct your family tree because they provide genealogy-rich information like names, relationships, approximate birth years, marital status, birthplaces, and parents’ birthplaces. Some details shed light on your ancestors’ immigration and naturalization, homeownership, literacy, education, childbearing history, neighborhood makeup, and more." FamilySearch Blog article
I love census records. There such a variety of questions asked that you will be surprised at the wealth of information you suddenly discover about your ancestor from World War I, as well as his family.
A patron once came to the Family History Center saying that her grandfather told them that he left home at 9 years of age and never returned. Imagine their surprise when he was recorded with his family at an older age. Did the great-grandparents lie and say he was at home? Did they not understand that only those actually living in the home are counted? Did great-grandpa exaggerate the age at which he "hit the road?" They will need to dig deeper to find out.
Most doughboys are found in the 1880 censuses and beyond. To find your doughboy, enter these terms into a search engine: "FName LName, State, 1880 (1890, 1900, 1910, and so on, but only one year at a time.) Census". The hit list will include databases that are repositories for digital images of the censuses. Have fun! WARNING: Very habit forming.