To me, that a viewer of any age can watch is practically the whole point of Khan Academy. Kids learn at different speeds and are interested in different subjects at different ages. Whatever subject you're talking about, there are some 8-year-olds who could benefit from it. The traditional approach of having grownups decide what material is suitable for what age is guaranteed to produce a mixture of kids who are bored and kids who are lost; khan materials present the promise that kids might be able to learn at their own pace and according to their own interests. The ones who find history of X fascinating will race ahead and do related research and file info away for later; the ones who lack context will probably be bored and lose interest and want to study something else and that's okay.