Well according to an 1985 pop song, "19" by Paul Hardcastle, the average age of a WWII combat soldier was 26, in Vietnam, he was 19. If a pop song isn't definitive, I don't know what is.
Here's a theory, maybe he's a "Hero" so he looks more experienced.
My main gripe is the lighting on the face. In those screen shots, it looks like there's light coming from under the helmet. It's pretty obvious the head was rendered without the helmet, allowing for the helmet to come off, hence the well lit forehead. It's not really a big deal, it just irks me as a photographer. I'd like to see the lighting adjusted when a helmet is on - but it's not exactly a high priority to me, and neither is how old they look.