The Percy Jackson movies are a bad representation of the story. In fact, the author hates the films. I was definitely shocked when I read the books and they were completely different! Thankfully though, the author was heavily involved in making the TV series, so we're going to see an adaptation that's closer to the books in terms of plot.
I'm glad they got actors the right age this time. The fact the film director made them late teens instead of twelve because he was worried it would 'be too much like Harry Potter otherwise'.

I was annoyed that the films aged up the characters and gave them completely different personalities. Especially Grover.
(Sorry - rant over.

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I think they change the ages of characters in movies and series to suit the target audience.