Walter Wiltfang age 22 and Ellen Nelson age 19 were married Oct 20, 1926 IN Brown County South Dakota Post Office was inWarner. The family is living in SD by 1910 census. So if he was in the service to would be from the state of SD. I don't find him listed in any Miltary databases at Ancestry or the NARA: http://aad.archives.gov/aad/display-partial-records.jsp?f=3475&mtch=9&q=wiltfang&cat=WR26&dt=893&tf=F&bc=slhttp://aad.archives.gov/aad/display-partial-records.jsp?f=3475&mtch=9&q=wiltfang&cat=WR26&dt=893&tf=F&bc=sl These are only Army Enlistments. If he was an only son or all this brothers were already serving, he was given an exemption in those situations. (My Family experience).
To be sure, if you know where he was living at the end of the war contact the local Veteran Organizations to see if he was a member. Check with County Clerk to see if a copy discharge paperwork (DD214) was ever filed with them. Did he claim Veteran status on property tax rolls? If you find nothing after checking then you can be pretty well assured he was not in the service.
The top age was 45 yrs for service and if a man was already enlisted in the National Guard or Reserve and his unit was called up for Federal Service and he was over 45 he stayed with unit and shipped out as far as I have been able to find out.