Marty,
I started shooting bows and arrows when I was 5, which was 53 years ago, and my first experience was in a group somewhat like the one you helped with. While I didn't have a "grumpy old man" help me out at that first shooting session, I sure would have liked it if someone like you had been there. I still remember the early group shooting sessions even though I don't recall the particular guys who helped teach me, and I am still shooting my bows regularly more than half a century later. I'm not sure what that kid is going to remember specifically about the man who helped him learn to shoot. I can guarantee you he will not think of you as "grumpy", and instead will remember the day someone helped him learn how to beat all the older kids. Hopefully, he will carry that success into every area of his life whether he continues shooting bows or not. As much as we all like to believe that archery is a lifetime activity for everyone, we are actually a tiny segment of the population. However, if archery helps that kid to be better in other areas of his life, then you have accomplished much more than teaching him to shoot a bow and arrow -- you have taught him that he can hit any target that he is willing to work for.
Good for you, Marty. I will pray for healing of your knee and shoulder. I have had both problems and know they are not fun to deal with.
Allan