Arthur is just a kid, and since we spend a lot of time peeking over his shoulder and accompanying him on his adventures, we get a kid's perspective on things. This means that while there's excitement and adventure aplenty, we don't get into super complicated stuff like, say, The Meaning Of Life. And while there are plot twists and risks, nothing in the book is so scary that we're going to have nightmares because of it.
What we're saying is, even if your age isn't quite into double digits yet, you should be fine with this book. And if you're into double-digits and beyond, this book should be a refreshing trip down memory lane to a time when things were simpler.