How old was Anna popplewell when she played susan in Narnia?

In the book 'Prince Caspian', Susan's exact age is not revealed. We know that she is young enough to still be attending boarding school, but considered by Aslan to be too old to return to Narnia after the adventure was completed. Anna Popplewell, the actress who played Susan Pevensie, was 19 years and 7 months old on the release date of the movie.