A military unit that historically ought to be composed primarily of young men in the 18-24 year range, is played by a bunch of actors who are at least a decade older than the part should require. It's not required the the actors literally be in their mid-30s, but that's the most common as it generally avoids Character Aged with the Actor
territory, and if the group is still in their 20s, even they are historically a little too old, it's not worth noting due to the subjectivity of how old someone looks. But a group of soldiers, sailors, or fliers who all, even the veteran captain/major/whatever, all look like they could be high school or college students is definitely an aversion.
Works that fit this trope include:
- Kelly's Heroes - a company composed mostly of privates, corporals, and sergeants, in the middle of World War II, seem to have an average age of somewhere in their mid thirties.
- In Gettysburg the use of reinactors for many of the the extras created something of a bias in favor of older middle age men, vs the younger 18-21 range that would have actually made up the bulk of the private soldiers.
- John Wayne in the Vietnam-era misstep The Green Berets - except he personally was more like a member of a late-fifties military unit. This in a movie set during a conflict where most of the men were only barely college-age, and the officers were decidedly unlikely to be all that much older.
- Many of the more accurate depictions of the WWII-era Red Army. Most of its personnel was from wartime drafts, and Russia threw just about every able-bodied male (and plenty of females too) they had into battle, hence their average soldier was older than a typical peacetime conscript.