I voted 7+ because that's the age for the "Kenpo-kids" class at our studio. We have one called the "Little Ninjas" for ages 4 through 6.
I've been assissting in/teaching the kids classes for almost 2 years. Actually, a more accurate way of putting it would be to say that I got suckered into it almost 2 years ago

. At first I wasn't too crazy about it but I've gotten to the point where I really enjoy it...most of the time. Some of the kids reinforce my belief that there are people who shouldn't be allowed to reproduce. OTOH, some of them are really awesome and take it seriously. There are a few that move better than some of the adults that have trained for the same length of time.
Our rank progression for the kids class is a little different, we use half steps once they reach yellow (eg. yellow-black stripe, orange, orange-black stripe, etc.). It is possible to get to "Junior Black belt" but it takes a couple of years, we don't have 8-year-old BB's that can't tie their own belts. The curiculum is also slightly different, we teach the same techniques but they only have to learn 4 or 5 for each level in addition to their other basics (and we don't emphasise the destructive nature of the tech.). We also do a lot of "fun" stuff like running-jump-kicks, padded sword work, and grappling, etc. they really enjoy this stuff.
We emphasise Respect, Focus, and Discipline and it "rubs off" on other parts of their lives. I've had a lot of the parents tell me that their kid's grades have improved and that he/she gets in less trouble than they did before they started training.
I think that the kids definately benefit from the program. For me, teaching the kids has been an exercise in patience. It's definately not for everyone but I do think kids programs have their place.