User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-4902593-20130115122705/@comment-3146930-20130123220101

I also think everyone is missing something kind of important...Percy is about a month away from turning 17, he is growing up. Percy Jackson and the Olympians followed Percy from 12 to 16, an age where most people try and hide from the future because they aren't sure what is going to happen. This is kind of the role of the Great Prophecy, as Percy will have to face the future. When he did face the future (completion of the prophecy), he now has to keep looking forward, and that means things like a job, college, and even a family.

Percy Jackson and the Olympians was about Percy being scared of the future, while in The Heroes of Olympus, he is looking to it. That may explain some of the change...or maybe I'm thinking to much about it.