I too have taught many hundreds of young people, although few if any as young as 6. If asked, my usual response is that 8 is a good age to start learning to shoot the bow, as opposed to playing around with a toy bow and suction cup or nerf tipped arrows. Of course, the archery equivalent of a Mozart does occasionally appear, so my mind remains open.
I always offer all participants in my classes the option of wearing or not wearing an arm guard. I also teach good archery form, which includes correctly gripping the bow. In most classes, someone who initially declined wearing an arm guard will change their mind after being spanked by the string a time or two. It doesn't take many spanks before someone might decide that there are more fun ways to spend their free time. People typically don't learn the correct way of shooting the bow the first time they are told, and there is no reason to make their mistakes painful. Also, there are those who because of body structure or choice of bows will hit their arms whether they are shooting using correct form or not, as evidenced by the widespread use of arm guards at all levels of proficiency.