Originally Posted by Prime Junta
Susan was condemned to perdition because she found out about boys.
Originally Posted by Wiki
Lewis's supporters also point out that the other children enter into the "new" Narnia (representative of the eternal Heaven) because they have died in a train accident, while Susan remains alive on our world, so that there is no proof that she has been permanently "excluded" (i.e., damned). The first footnote under Susan’s entry in Companion of Narnia by Paul F. Ford helps in an understanding of Susan’s absence at the end of The Last Battle. Most importantly, Aslan’s last words at the coronation in The Lion, the Witch and the Wardrobe offer the best justification for Susan's eventual reunion with her family and friends in Aslan’s Country: "Once a King or Queen in Narnia Always a King or Queen in Narnia…"