An interesting question. The editors of the magazines Bradbury published in took care to not improperly influence the young, and their readership was quite young. Given that, and the earlier age at which children are exposed to things today, I would say early adolescence--12 to 13. That is old enough to not be frightened by what they read...but young enough to remember being like the kids in the story.
actually, the veldth is programmedfor high school students which means someone between 15 and 18