From my limited understanding, generally police-type agencies partner an experienced senior agent/detective with a junior one.
However, at the end of "Men In Black" movie, Agent K announces that he's ready to retire and he has been training Agent J as replacement:
Now, Agent J is a brand new green rookie - he was hired just a couple of days before (based on my understanding of movies' timeline); and doesn't know anything; see the scenes with Agent K explains obvious beginner type stuff to J, including how to identify a bug, or general methods and mission of MIB, or hell, the starting-off on the job:
Kay: Cool, whatever you say, slick, but I need to tell you something about all your skills. As of right now, they mean precisely... dick.
So, how come Agent J ends up being the senior partner once Agent K retires, instead of being partnered up with another senior agent till he stops being such a rookie?
NOTE: I'm specifically asking about the very end of MIB1, where Agent J is a senior partner to agent L - not about eventual MIB2 where J is obviously more experienced and more likely qualifies to be a senior partner.