Since they are doing the Magician's Nephew it does make since to portray a younger Jadis since this is when they first introduce her in Charn. It will also be a few more years of acting needed for her if they are doing TLW&W The Horse and His Boy and Caspian. The first and last of which she makes appearances in. She's only immortal when she eats the apple in Narnia.

She's old enough to play Jadis, and if she's doing The Magician's Nephew we would see her in Charn, then later if there's a remake of The Lion, The Witch, and the Wardrobe. I think she could pull off evil if she tried.