Just an odd thought that occurred to me the other day:
A Prussian born in the 1850s would have been ~20 years old during the Franco-Prussian War. In 1940s that same man would be in his 90s.
Similarly, a Russian man born in the early 1860s would be in his late teens during the Russo-Turkish War of 1877-78 and would be approaching 80 years old in 1940.
Obviously, such men would be WAY too old to fight on the front lines under normal circumstances... but remember 1) the formation of the Volkssturm militia and the desperation of the Nazi regime towards the end of the war and 2) the anti-Nazi Partisan groups resisting the initial German invasion were made up of local inhabitants from various Eastern European villages/cities rather than uniformed soldiers with age-limits.
With these things in mind, are there any records of elderly men who previously took part in major 19th century wars also fighting in World War 2?