First off, the video in the media is not mine, but is also a great dragneel thory you might want to take into consideration.

Anyways...

ok we all know Natsu is a Dragneel, along with Zeref and Larcade.

Basically this is a redo of the last dragneel thory, because i thought of way better one!

Larcade is another brother of the dragneels. He isnt the dad, im pretty sure he died. Larcade is working under zeref, and it would be pretty weird to have your dad work under your command. Larcade was probably not in the fire. He might have been in school, in which case he is either the oldest or middle child (im assuming middle), or just out with his friends when the fire started. He probably came back to his burnt down house, seeing nobody around. He may have assumed his family to all be dead, which makes for a good family reunion with zeref.

He probably worked with some other people to get ahead 400 years in time, or also has a curse. My guess is he lived his life after his parents died for, hmm, let's say 15-20 years? Then he met up with his brother, Zeref, when Zeref started making the empire. I doubt Larcade knew he was brothers with him, Larcade might not even know now. Zeref might have kept the fact that they're brothers to himself. Then Zeref, being the powerful wizard he is, gives Larcade a spell/curse that makes it so he wont age. It might even be the same curse as Zeref's, just a light version. Larcade is known to have light magic, brightness, etc.

Anyways, that's all i got! 

(Also sorry for not updating heheh ive been super busy!)