I don't know. But digital design concepts are not that difficult. I don't think there really is a minimum age. Digital design can be taught as a game of building puzzles. Almost like legos. A young kid should be able to pick the basic concepts, I think. You could treat each of the gates as a lego block and go from there.
I think that if you can teach a young kid programming concepts then you should definitely be able to teach them digital design concepts.
I think that if you can teach a young kid programming concepts then you should definitely be able to teach them digital design concepts.