When you've served in the front lines of a major world war, facing death multiple times a day, possibly watching friends get blown up before your eyes -- you have my blessing to find and marry a woman in two weeks

Some things accelerate the maturity process. Facing death constantly is one of them.
I'm sure those GIs were very mature after their experience.
As an ex-seminarian, I can relate to some extent. In the spiritual realm, I was basically in boot camp during a time of war. We all knew that souls were being lost, and that the world was in grave danger. We had to use our time wisely and study and pray a lot, because we never knew what we'd be called upon to be knowledgeable about.
I strongly believe that 3 years in seminary during "normal times" is NOT the same thing as 3 years during a crisis. It's like being in the military for 3 years vs. being in the military for 3 years during World War II. There's a certain "urgency" in the latter case, a driving sense of purpose.
It's very hard to go back to the world after completely leaving it behind for 3 1/2 years. That's why soldiers experience depression and other issues re-integrating with the happy-go-lucky world after the war ends.