Lorraine I agree- 5th and up.... My 10 yr old's teacher just finished playing the audio book version of this book for his 5th grade class. This is the first I heard of this book, and my son (who hates to read) liked it so much that he wanted to get it out from the library. He has listened to the CDs so many times in the past month. He had me listen to it also, and it was a really good book. He now wants to get the rest of the books in this series! If it was inappropriate, his teacher would not have been playing it for his class during the school day.
Madalyn I agree with Kelly, however, as you go on in the series the vocabulary and understanding of what's going on tends to rise. I would recommend this series for 5th graders and up.
Kelly While The Prisoner of Cell 25 could be included in an elementary library, it is more suited for middle and high school readers. There is not content that is directly inappropriate for elementary students, but the themes are intended for an older audience.