User blog comment:Sargenttacos115/Natsu and Gajeel age Theory/@comment-94.3.180.232-20140217174527/@comment-7418318-20140504093701

Zeref was alive and awake during those entire 400 years. Well... not the entire time. I'm sure he still slept a certain set of hours a day like any other Human, but there was no coma, sealing or timetravel on Zeref's part. He just lived in hiding to avoid anyone being killed by his out of control magic.

So why does Natsu have to be from 400 years ago for Zeref to know him? Zeref could have run into Natsu on practically any day from 400 years ago to right the day before Igneel left 7 years before they met on Tenrou Island. With his heartbreak over being split from his foster father, Natsu surely wouldn't remember some stranger passing by the say before.