Why everyone in this film so old?

I would love it if 53 years old women had bodies like Mitzi Gaynor's...

The pilot guy looked young, at least! And Mitzi didn't look a day over 35!

Seriously, Hollywood has never dared to cast soldiers as young as they really are. For most of the wars in the history of the world, and for WWII specifically, most of the troops were made up of menaged18-28. In WWII males aged 18-37 could be drafted, but as a man with dependent children could beg out, that really meant that much of the army was made up of men and BOYS aged 18-25.

But for some reason, aging male stars have always liked to play soldiers, so there's all these film featuring a 50+ John Wayne or Clark Gable, playing a soldier or officer who would have been 25 in real life.