Well, you could look at it as a practical vs theoretical argument. I look at it as a realistic vs unrealistic argument. I suppose my view is colored by my actual experience in the military. I should really be more open-minded.True. However technically speaking, there isn't an age restriction on the rank and one can theoretically reach that post at a much earlier age than as practically occurs. Plus, given the increasing amount of 'field time' soldiers are receiving, it is thus believable for the general public to assume that Tatum's character attains this by being 'the best there ever was' (if they push the idea).
Seriously, your last argument about Duke possibly not even being a 1ST SGT is more on target I think.