I believed in Santa until around age 9 or 10. My brother revealed to me a few years later (who at the time was the same age I was) that he stopped believing around 5 but put on a good front. I didn't read this one before then, but I had read other books where it was hinted that he wasn't. It was sort of like sheesh, these are directed at elementary school kids, some of whom still probably believe. Though I got into a debate around last Christmas about this topic and the general consensus was, it's okay to say these kind of things and for authors to include them in books because just because kids read them or hear people say he's not real, doesn't mean they'll stop believing. I sort of believe in that, because we had jerky kids even in the earlier grades saying that he wasn't real and we were just kinda like "Whatever you say, you're getting coal." Still doesn't mean Ann/ghostwriter should've included it.
Funny you say this, as the way I found out about Santa (and etc) was because of Claudia's Portrait book!! I was devastated.