User blog comment:Miskos3/Fairy Tail 514: Dragon Seed, Review/@comment-25324252-20161213225834/@comment-2214433-20161214065241

Well, for starters, Freed's runes may have reacted to the dragons inside Natsu, not Natsu himself, as the dragon slayers aren't over 400 years old, not biologically. The same can be said about Erza.

That, or Irene could've time travelled before Erza was born.