How?

In Harry Potter and the Half-Blood Prince, Harry has grown up, and the book is a little more grown up too.  One of the things I love about J.K. Rowling is that she writes with depth, she doesn't play it down for children.  But even with that, the series matches Harry so the first book is much more magical (which is why it's my favorite) while the later books are older and more real.  Rowling never writes one dimensional characters, but we get to see more because Harry is old enough to understand more.

Ok I've blabbed on long enough here's the quick summary.

"But he understood at last what Dumbledore had been trying to tell him.  It was, he thought, the difference between being dragged into the arena to face a battle to the death and walking into the arena with your head held high" 

Accepting your fate...that's kids stuff