I'm trying to figure out why revealing Mel's age is significant. Maybe it is foreshadowing that her resurrection of Jon Snow is going to require so much power that it will kill her, and she knows it.
It is not shocking to me that she is really old - she's basically a witch so faking her own "youngness" isn't a surprise. The fact that her body actually looks that old is a little jarring.
Right now she is questioning her whole belief system - the Lord of Light didn't save Stannis even after she sacrificed Shireen. Which, BTW, the killing of one little girl is less significant to her because she is so old and has seen lots of people of all ages die in her lifetime.