User blog comment:BlackWizardZeref/Fairy Tail: Chapter 380 Hell's Core/@comment-76.124.155.27-20140413200702/@comment-24039250-20140413220519

Zeref wasn't the one who created Fairy Tail, however; that doesn't mean that Zeref didn't inspire Mavis to want to create Fairy Tail. I feel that Fairy Tail's True purpose is to in the end fight Zeref, and more importantly to show him that the Era of the "Golden Age of Magic" where wars will end and peace can finally be born is coming, and this guild will herald that Era. Mavis and Zeref appear to be very close, whether its past allies, siblings, or even lovers, so I feel that Mavis wants to show Zeref that the humans can learn peace and that they do not need to die in order for peace to finally occur.

Also the whole "Natsu and Gajeel over 80 thing" I have a simple theory for that one. In either this or next arc, I see a few members of Fairy Tail going to the 400 year past, and because they would exist in a time period that is 400 years ago, the runes would considered them over 80 years old.