Here is John V Plantz's appearances in censuses over the course of his life, up to 1900, and a listing of his appearances in obituaries for his parents back in the Mohawk Valley.
The compiler has not included John V Plantz's appearances in the Colorado Census, nor has the compiler included references to his grave marker in Colorado.
So something is odd, but very often something is odd with census records. It takes a little attention to notice just _how_ odd this is.
(1) The census entry which the tree listed above mentions when John and George are 18 clearly labels two boys as twins: there's a backeting mark and the word "twin" written vertically across the lines.
(2) The boys in question are "George" and "Henry", yet the boys in question are "George H" and "John V" in the previous census.
(3) The 1900 census record has John Veeder giving a July 1842 birth date (compatible with the first two census entries, but nothing in between). In 1900, George is living in the same Iowa county as John.
(4) The twins turned 21 in 1863. The first Union-side draft occurred in 1862, but the first real effort started in 1863. George Henry mustered in and served until the end of the war. John told two rounds of census takers (and probably the rest of his life telling his kids and grandkids) that he was a couple years younger than someone born in 1842 would be. (If you're wondering about the 1890 census, go read the sad, sad tale in wikipedia and then be amazed at how long ago Hollerith was.)
(5) John V appears in the 1910 and 1920 census, giving an age compatible with a July 1842 birth date and living in Colorado. Some of his family came with him, but others stayed in Iowa.
Sidney died young. Sarah and Earl both married but I don't think they had children. Bernard had kids and stayed in the Mohawk Valley. George married late, moved to Audubon, but returned to Mohawk to be buried, as near as I can tell. He didn't have children. The offspring of Bernard and their genealogically oriented membership and the offspring of John Veeder and their genealogically oriented membership appear to have had absolutely nothing to do with each other thereafter. And John Veeder's descendants appear content with what really looks like an inaccurate birth _year_. (Heck, I was until a couple days ago.)
If Bernard has descendants in my age range, they are my fourth cousins. I'm reasonably certain I've identified descendants in my parents generation (3rd cousins once removed) who live not far from where all of this happened in the Mohawk Valley, and quite close to where I spent Christmas at my sister-in-law's house.
If you're researching Adam and Maria's kids and would like to add your opinion, I'd _love_ to hear it. (And if you're tempted to attack my character, just pay some attention to how careful I've been to not come right out and accuse John V. of anything other telling some census takers one thing and other census takers other things.) Census records can be screwed up. I was prepared to believe the moving birth year was just one of those things, until I noticed that there might be another explanation.