I became a firearms instructor with a little over a year on at a federal agency, howevere I did have about 7 years of other LEO experience from outside that agency before that.
I can say this.....fed agencies are a lot smaller than some state and local the canidate pool for instructors is more about who wants to do the job (which is typically an added duty to your normal work load with no extra pay), who has a passion for the subject and who is competant enough to learn it,/pass training and has the ability to teach it.
We had a few older folks on the range nearing retirement who "knew it all" since they were on the job before I was born. Their qual scores did not show it. I gave them the advice they needed. A few took it and improved and realized the kid knows his crap. A few didn't. Their scores were submitted without any extra "help." They talked about the "brotherhood" and other stuff....yet when I threw back as a brother they need to have my back, and being able to shoot is one of those things, they had nothing to say. After a second quarter of not listening to my advice and poor scores, HQ advised them they were going back to FLETC for a week for a refresher class. All in their late 50's came running to me, asking me to help them and re-qual them and get them out of going to the class. I did...they listened....they didn't have to go. I don't believe time on the job makes a good instructor. Don't get me wrong...it helps...but I think theres other things that contribute.
-In God we trust. All others, put your hands on the car and don't move.