User blog comment:Sargenttacos115/Natsu and Gajeel age Theory/@comment-73.186.255.134-20140731024610/@comment-98.116.156.226-20140811150428

Also we are missing Wendy here... She is in the same boat as Natsu and Gajeel. Her dragon also went missing on the year X777. So it shows that Zeref would have to send all the real dragon slayers through the portal to be able to defeat him in the future. Which would be impossible if the portal was not invented yet.