So, I'm going to cheat a little. This review is of the whole Percy Jackson and the Olympians series, not just one book. It's easier that way - and I catch up some.
This is a series of 5 books by Rick Riordan. Percy starts out at 12 and the series ends when he turns 16. Overall I have to say I liked the books. Keep in mind they are aimed at kids, not adults. If you go into it looking for an adult level reading experience, you will be disappointed. But, from a young teenage perspective (which is the intended audience), this is a great series. It is fast paced, fairly heavy on the action and a fun subject.
I would say my biggest negative critique is that I don't know if the character development across the series was quite enough. To go from 12 to 16 in the series, it felt a little like Percy didn't grow up much despite getting older. Maybe that's just me, though. Like I said, these are intended to be kids books.
In the end, I though the series was a fun read. That, to me, is the most important since it may help inspire kids to read more. I would recommend it to most adults looking for a pure, simple, fun set of books to read as well as most kids.