It's really contrary to United States military philosophy to put someone in command that hasn't had leadership training + experience at the minimum. Even in Vietnam, the average age of someone in the enlisted ranks was still twenty. The youngest company CPT that I know of was 21 years old and he was a highly decorated, exceptional soldier.
Therefore, I would chalk it up to plot necessity and not look at the issue very deeply. Otherwise, Sirius would be a twenty-something year old butter-bar LT posing as an exchange high school student or, more likely, an English teacher.
