In the first book, Percy felt too old for his stated eleven years. In this book, the second in the series, he feels younger, and that is a good thing. I didn't have any trouble believing he was only twelve. At times, he might have seemed about thirteen, but I could live with that.
This was a marvelous tale. There was a little bit of everything for everyone. I do like the ambiguity of some of the more villainous heros, such as Clarisse and even Luke. You may not like them, but Riordan does allow us to see why they act as they do.
There are shades of Harry Potter in these books, but that's only fair: Rowlings borrowed heavily from Greek and Roman mythology.