I want to recommend the 'Song of Ice and Fire' series (George R. R. Martin), but it's awfully long. I haven't read all of the books myself yet, but there's definitely a lot of character development, and I predict a couple of characters will be 'bad-guys turned good'.
You might enjoy the 'Enders Game' series (Orson Scott Card).
It does start off with Ender as a child, but it's pretty difficult to remember that he isn't an adult anyway, and we get to see him grow up