Please don't read if you haven't read the latest chapter...or any of it at all.
YOU HAVE BEEN WARNED.
So. Er... Erza has turned into a child. This is probably an effect to the mountain...or is it a mountain?
I think the mountain IS the ETERNAL FLAME. It would make sense, in a way. Seeing how Erza's logic tied in with a few other points. The giants are, well, giant, so that must mean the flame must be huge as well, or else one tiny step on it and -BAM- gone forever. Most likely its the mountain, to me.
About the child problem. I took a second to go "AWWWW!!!! ^_^" before realizing that was a huge problem. Back to my point about the eternal flame... What if those changes are permanent, hence the word "eternal". But I highly doubt it, though I believe those effects aren't going to wear off anytime soon. It may have to do with the ice and flame mixing together, but what do I know?
Ultear + moutain that Erza's at = youth?
I don't know. It could be possible that she would be able to retain her youth back, but I doubt she would. Mainly because she is completely satisfied and she doesn't need anything else. You saw how she was a few chapters ago.
Poor Ultear. She thought her life was a waste, but she saved mages from their death. Brilliant mage.