By the end of Prince Caspian, Susan and Peter are too old to return to Narnia. They are told that they have learnt all they can from the world of Narnia, and that they must now seek Aslan's identity...
Susan and Peter are "too old for Narnia" after Prince Caspian, be it the movies or the books. However, if they do reappear in the third movie, it would be a minor point, as it would be outside of...
Aslan told Susan and Peter at the conclusion of Prince Caspian that they had grown too old for adventures in Narnia and that they needed to learn to live in their own world.