The Youngest Of The Five Men Is The Major?
Is that common practice in war time? Wouldn't it cause resentment among older, more experienced men to be told what to do by a younger man?
This was a common situation in WWII. The draft age was 18-35 for most of the war (up to 45 for a short time, too). So you could have some enlisted man in their 30's commanded by an officer in their early 20's. An officer could be a recent college graduate or a "90 Day Wonder" from Officer Canidate School. See https://en.wikipedia.org/wiki/Officer_Candidate_School_(United_States_Army)#World_War_II_era
And, officers in WWII were quickly promoted with the ever-expanding military, so it would not be unusual to see a major or even colonel in their mid-20s, especially if they were exceptional peformers. An example of this, appropriate for this site, is Jimmy Stewart, who was commissioned as a second lieutenant on January 19, 1942 and was a full colonel by early 1945.
Yes, this could cause friction between young inexperienced officers and much older and experienced sergeants. This was especially true in the Vietnam War where officers were rotated in and out of units in a short time just to punch the combat experience ticket for higher promotion.