In episode one of season six there is a scene where the Red Woman (Melisandre) removes her clothes and looks at her reflection.
See in this clip,
My take on this scene was that she has done some really horrible things for the Lord of Light and she for the first time in the series feels conscious about what she has done. So when she looks in the mirror it shows her inner self which is ugly from all the terrible things she has done up until now. Of coarse that is just my thoughts and are probably wrong.
It obliviously is never explained in the show, but What is the message we are supposed to get from this? What does this mean? Why does she look old and frail?
Edit: For those in the comments who think the necklace is the source of her younger looks, she has taken a bath before and not had it on, so it can't be the necklace that makes her look young.