I play with all my little brothers – they're 12 , 9 , and 5 we. We play D&D 3.5 and we have a blast.
I think if a child is somewhat intelligent and can roleplaynroleplay fairly well then he or she is ready. My 5 year old-year-old brother is by far the best role playerroleplayer in my small group. I think little kids are awesome to play with. justJust don't burden them with rules at first – slowly add the rules, focus more on the rproleplay, and they will be on the path to becoming epic role playersroleplayers.
Hope you have many great adventures!!