Perfect for preschool age all the way up; even parents can learn
My son has been watching since he was 3 he is now 4 going on 5 and has an overwhelming amount of knowledge on his sholders... If you don't have access to hbo I would reccomend getting it like yesterday!
Our first grader LOVES this show. The content is mostly above her grade level, but she learns a lot and is compelled to watch it all. If you can record/DVR/TIVO it's a great asset. You can pause the questions and quizzes and give younger viewers a chance to figure out the answers. Great fun for the whole family. A breath of fresh air.