User blog comment:Sargenttacos115/Natsu and Gajeel age Theory/@comment-121.54.22.108-20140105171117/@comment-67.167.56.174-20140106191230

Also if you watch the Tenourjima Arc in one of the 1st couple episodes of the second part of the exam Zeref tells natsu that, "he has grown nice and strong" and he says that he has wanted to see natsu again. So you're theory of them being from the past is quite plausible.