Quote from atem4yami
The reason why he showed up 400 years ago was to kill the dragons in the first place. That explains why he was there. And Zeref being too young? When you've been alive for 400+ years, you pick up a few tricks. Also something could have happened around the time he was sealed. There's 400 years of mis ...
You are stretching it by making such assumption. You are adding history that is simply not there. There has been no hint of magic that can change appearance or of Zeref possess it, myth or not.And if he is sealed for 400 years then technically he has not been alive for 400+ years in terms of experience.
It might be a decent theory if this was the last arc, but I think there will be more Fairy Tail and Zeref's origins will eventually be discussed later without a connection to Rogue and Sting or even anyone else already in the series.
Why would he go 400 years into the past to kill the dragons. If he could do that why would he show up two days before the event and not go back 14 years to before the dragons disappeared. I know eclipse is an important part to killing the dragons, but he is still cutting it a little close. 400 years seems like too great a stretch in time.