She also uses some form of magic to stay young. You might believe it's that ruby red necklace, I think she bathes in in the blood of newborn babies, or something equally Wicked Witch-ish.
To complicate things, there might even be another Red Woman making her presence known in upcoming episodes. A mysterious woman named Kinvara showed up in Meereen this season wearing the same ruby red necklace! Does she believe that Daenerys Targaryen is Azor Ahai?