Though she was typically seen with a youthful appearance, Melisandre’s form would change when she removed her gold choker necklace with the glowing ruby.
It turns out she was actually a very old woman, but the power of the necklace transformed her body and made her appear and possibly even feel younger.
The thought that Melisandre could be alive in a series that goes as far back as the prequel is rooted in the mystery surrounding the character’s age.