I would think the majority of the able bodied males would have fought in World War 1. World War 2, only being a short time later, would create a sort of a gap in available men to fight. Correct?
WWII started 21 years after WWI ended. So the men fighting in the second world war are those that were born during the first. While there was a drop in the birthrate during WWI, I am not sure if this somehow affected Nazi draft policy.
[Source (pdf, German)] (http://www.zdwa.de/zdwa/artikel/broschuere/broschuere_gesamt.pdf) The birthrates are on slide 6, the blue line is number of birth per 1000 inhabitants and the orange line is number of death per 1000 inhabitants. The world wars are the sharp upticks in the death rates.