As someone who has been castigated for what was said when those doing the castigating were not there to see and hear the totality of te interaction, I have a few comments for you and for anyone who wants to add their opinion to the discussion.
First, it is not so much what you say (unless you use "fighting words") as it is how you say it.
Second, armed or not you, IMHO, have at least a moral/ethical duty to try to preserve some of that thin veneer of civilization that keeps us from just clubbing the living daylights out of the rude and crude folks who think they are entitled to the head of the line because <insert asanine reason here>, as opposed to asking if they may be allowed to go ahead of you. As noted above, it is more about how you fulfill that moral/ethical duty than actually doing so. Being just as boorish as they are does not help the cause of civilization or, if you are bearing arms at the moment, the cause of RKBA.
Third, if it takes the overwhelming force of arms, or the perceived threat of the overwhelming force of arms, to get the young protagonist to consider the possible error of his ways, then civilization probably needs to chalk this one up in the loss column even if he did go to the back of the line and wait his turn. Had he, instead of staring at your hogleg, looked at you and the other people in line and hung his head in shame and uttered an apology for being crude, then I would say civilization could chalk it up as a win. All this is said with Marko Kloos' wonderful essay "The Gun as Civilization" (google Major Caudil, that plagarizing bast*rd, to read the essay) well in mind.
Fourth, IMNSHO you did right and good and what was proper. See #1 - #3 above as to why. Even so, our young protagonist could, if he were smart enough to figure it out, could have entangled your life via a complaint of brandishing. To avoid or counter that is why many of us recommend the carrying and use of digital voice recorders at all times we are outside the house, and even going to the point of asking those behind you, and the clerk, for at least their names as witnesses. No need to get out the old notebook and pen when the DVR is running. It's also a good opportunity to engage others in conversation about RKBA and how this event demonstrates its effectiveness without ever needing to put your hand near your handgun. (I know that last sort of flies in the face of a lot I wrote above. So what? Deal with it!)
stay safe.