We don't know exactly, but we think she may be somewhere around the age of nine or ten, since in the plot summaries of the movie it states that she is six in one plot summary, which she clearly is not, and is a pre-teen in the other plot summary, which she clearly is not. If you look up the biography for Victoire Thivisol, it states that she was born in 1991, and therefore would be around age nine or ten during the production of this movie.
As for verification of the previous question, Anouk is six-years-old. If you have read the novel by Joanne Harris, you will see that she intended for Anouk to be six years of age, and although the Anouk in the movie does not appear to be six (just one of my problems with the movie) she is supposed to be six years old, according to the real story.
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