I do not agree with much that has been said about this show in general.
Though there at at least two or three characters who are mostly very vain and are very girly, the other characters in this show do not display these aspects. When one character does for an episode, against her usual habits, to impress a boy; she learns that its more important to be true to yourself than to change yourself for someone else. I think that is a positive message that older kids should not overlook.
When a girl and her horse come to horseland, you learn she is Native American. She is later offended by two of the other girls, who have wrong preconceptions about Native Americans. At the end, the girls apologize, and it is clear that what they did wasn't how you should act.
I think a child should be around 8, when they watch this show, so that they don't just pick up the negative messages, but they pick up the good ones in the end... Make sure your child would notice that everyone made up and all issues were resolved, and that they pay attention to the manner in which they were solved.