Im gonna rant for a minute, so please excuses my pigheadedness I have argued this point in other discussions on this site, but adults gotta realize-kids ain't stoopid. While I agree that most u13's don't have the ability to teach class, many will surprise you. As aforementioned, maturity and ability should be determined on a case by case basis. As for those that say a small person wouldn't be able to do throws and what not to larger people, are you min the right style? If I were learning or teaching such things I would want to be sure the technique worked on folks of all sizes. If you're training in a style that doesn't teach you how to take town bigger folks, well, you're dead. And junior blackbelts? Really? Why junior ranks at all? Is this not the grandest form of watering down a style? Why not go ahead and hold kids to a higher standard? Adults hold kids to a higher standard all the time, now suddenly you want to hold a child down? At something this child presumably loves to do. For what reason? If you hold children to a different, lower ranking system than adults, of course you'll never want to learn from them. If you hold kids below your level how can you expect ever to surpass anyone? All that being said, I believe your instructor should at least be able to get them self to class. So the minimum age for a solo instructor should be sixteen(or drivers license age). One cannot learn to teach unless given the chance.