I don't think Kirk will be a "cadet" for very long either. Some establishing scenes of him as a cadet after "the bar fight" to get him into Starfleet, the grad ceremony/assignments (in red)--then a flash-forward 3-5 years where Kirk is now a Lt. or Lt.-Com. Chekov could be an ensign in that scenario (thus preserving his "jr." status vis a vis Kirk) and simply be on the Enterprise before Kirk without being "older/as old" (clearly NOT the case anyway, just by virtue of the casting) or senior/equal in rank to Kirk.
It seems that so many are willing to assume that the least likely presentations will be the ones we're given in the film ("cadet to captain in six minutes", "Chekov outranks Kirk", and so on). Why assume the least likely to be true in this case when such a position, in general, is absurd?