One of the most exciting things about the films would be to see the cast throughout the seven films, and to see them grow throughout their years at Hogwarts. That may be a pipe dream, that may not be possible, but it would be, from a cinematic point of view, very exciting. You would actually see Harry Potter grow from an eleven year old into a seventeen year old.
Chris Columbus during the filming of Philosopher’s Stone (Capturing the Stone: A Conversation with the Filmakers)
Emma Watson | 'Harry Potter and the Half-Blood Prince' UK premiere