The age range is around 13 to 25 for the chosen one's while the guide is 25 to 30. Do you have a rough idea of how many people do you want to do?
It's not necessary to read it, but I do recommend it simply because it is amazing, and reading the it will give you feel for where I'm coming from and what I envision for the series.

Also there are two possible way the story can begin. We can follow the group an elite group or a fail group. Personally i prefer the fail seeing as they are generally easier in terms of personality, but I can go either way really.