okay, even if sepranttailedangel destroyed my idea about the age of natsu and gajeel(sorry so far, but i didnt read that Q&A, neither i heared about it till now xD)
some of my ideas dont really need the fact that natsu has to be over 400 years

. its more like that they could even fit more now. its gonna be a whole lot of some information we know about, mixed with some thoughts of my own.
i dont think that zeref is an actual dragon slayer. the facts we already know about zeref is his magic of life (zerefs book, demon creation and taking life away and so on) he somehow was on tenroujima iland(what i think wasn´t by any random chance). he knows natsu for some reasons. he somehow sealed himself inside his body. he is immortal, he was mostly the guy who transformed a person into acnologia.
that are some facts we know. considering what you said Serprant, that thing with the life absorbtion... well even if zeref was "asleep", lets just call it like that, death waves were coming out of him in a random pattern, taking everythings life away. well that could also be connected to his imortal life, by taking away the life of other "living" things to add that ammount to his own life. but for that later more.
we know that zeref was mostly the man who "created" acnologia. that happened 400 years ago and i still think that zeref wants to stand up and stay for his crimes. well, in the end we dont really know why he turned that man into acnologia. since acnologia started on the human- liking- side, would mean that zeref and the human-acnologia were be friends or something like that. i mean, you need a got damn good reason to turn someone into a dragon. from this point we can only guess what really happend. maybe after acnologia recognized that bathing in blood makes him stronger, even after beginning to murder dragons on his side, something pretty important must have happened. but also here, later more...
what part plays the dragons - king - festival in that? acnologia himself was the king of dragons (despite beeing a human to begin with)... well... they say the grand magic games orginally went by the name dragons - king - festival, so did they perform some kind of games back then too? they said, that dragon, demons and humans cant coexist in one world. would that mean there were 3 teams? humans, demons and dragons? but also here i got a new idea how that festival has his roots, but for that

guess? later more ^^
then there are the god slayers aswell, where my guess is that zeref created them, for the same reason i already written down. the god slayers magic is kinda similar to zerefs magic, so it would fit that he created them bach then 400 years ago (well otherwise it wouldnt be a lost magic, since zeref seemed to disappeared in that time till the present) and at the same time god slaying magic is pretty similar to dragon slaying magic (eating elements like air)
since the S-Class arc we were all woundering about the connection between natsu and zeref. why the heck does he know natsu (even without my 400 year natsu idea)? and for what reason is that page:
http://www.mangapanda.com/fairy-tail/253/17 why the hell did he say, i"ts over, isnt it?" and shredding tears... like the end of hope? this is also an important thing in my new idea.
i recently re-read some chapter that i liked and found that:
http://www.mangapanda.com/135-44553-18/fairy-tail/chapter-166.html
is he talking about pollyushka? well, doesnt really matter; we know that someone made something like a prophecy about ft... that when 3 dragon slayers are in... fate is going to make his move...
and its all about that in the future they lost to fate on july 7th. and after the one scene of igneel, about that the time is becoming close when the dragons have make theire move... could he talked about july 7th in the grand magic games? he also talked about the dragons of sting and rouge in a way, as if he could possible watch the happenings at the grand magic games(about the fight thats gonna happen Natsu/gajeel vs sting and roughe). is it just me who thinks so?
and then we got the future lucy

sorry for listing some details, but they all come together^^.
she said that the games would take an impossible outcome and when that happens the princess opens the gates of eclipse. so, the most funny part is about to happen, the part that makes everybody, every single time it happens confuse... time traveling
i know that my ideas now get a bit "crazy", but endure it pls
so we have 3 timelines: past, present and future.
the one and only reason zeref knows natsu is that they already met. the question is not where... its most likely when did they met? s
since the present natsu doesnt knows zeref, could it be that a time traveling natsu has shown off in the past? could it be that zeref made a promise to natsu back then, that in the present he will change something? that would explain the tears of zeref as he watched how acnologia destroyed tenroujima iland. cause since zerefs believes that natsu is dead, he cant fullfill his promis and it will happen again...
still i dont think acnologia didnt appeared on tenroujima by accidant. after the "inner zeref" awaked and get rid of GH, the dragons king shows up? sure....
so, since zeref created acnologia, acnologia surely can sense his magic. that must be why the dragon appeared on that iland after zeref awakened. at the moment he wants to punish GH, he says some very intresting stuff. it also proves that he changed back at the time 400 years ago. he recieved a curse? well being imortal and the idea with the lifetaking to live on,.... when he fights for the importance of live... well i would call that a curse. its like that curse was casted by himself on himself, that he could live on... well more that he must live on.
more; acnologia calls for a change of an era... the end of the human era would be a start^^. he said they pulled the trigger for all that, which should mean by "awakening" zeref they pulled the trigger.
so here something about time traveling that mostly every single time happens. with the idea to travel to the past and try to correct something, you mostly pull the trigger that this will happen. its a bit of a stupid example, but lets look at the movie Terminator. the whole thing with traveling through time is the fact that the future is about to happen. so i think that the future lucy pulled the trigger by traveling through time that everthing will happen as it does in her timeline.
so all in all together: in Zerefs timeline where the war between the dragons and humans is ongoing, someone of the future appeared. i dont think that they said: hey lets make a tournament where we are fighting each other for the domination of a species. it was got damn war. no one thinks about a tournament in such a case. then, someone appeared from the future and interveres. they dont belong to that time, and they hell shouldnt be there.
zeref gets information he should never get and so by abusing eclipse and traveling back 400 years, they make the big mistake, not also that they are now missing in theire own timeline...
one person who must have traveled through time should be natsu. otherwise he and zeref would have never met. but something must have happened to natsu then, while zeref was mostly like forced to curse himself to live on till the time natsu is born(present).
its said that zeref threw the world into chaos, maybe thanks to creating acnologia
so, back to the present again: zeref was on tenroujima iland and met finally natsu. natsus first impression, punch in his face. but then zeref said something intresting again. you still cant break me?
if we consider thats the zeref from the past 400 years, and saying still cant break me... this could only mean that in zerefs time a natsu showed off and fighted with him.
but why is he saying that natsu has grown? it could only mean that zeref saw natsu while he was little. and natsus muffler is affected by zerefs magic? how comes? could that muffler be from zeref himself? to protect him from his own magic? would fit in my opinion since natsu never saw how his dragon dissapeared, just that at one day he was gone, everthing he left was that muffler. well... if you think about this 2 timelines. past and present, with a past zeref who got informations from the future that he never should recieve... when zeref was friendly with the dragons and wanted to fight for the importance of life and met someone he never really should(natsu).... he learned about the importance of live and changed ( thanks to natsu

)
that it was zeref who told igneel about what is going to happen? that igneel has to teach a boy called natsu born 400 years into the future the art of dragon slaying, so that the presence fits with what happend in the past. that in the future on july 7 7 777 the dragon of this kid dissapeared? how else should it be that igneel suddenly exactly in that tournament is going to make his move, when there were many in others during the time where ft mages are frozen in fairy´s sphere?
maybe cause they have to, cause its already happend once and they must to save timeline.
maybe zeref gave this muffler to igneel that he can give it later to natsu to protect him.
it would really explain why the dragons dissapeard without any reasons... cause they know they had to. when they didnt... the timeflow would be disrupted and everything possible that could save the world from acnologia is never gonna happen(the dragon slayers).
it was zeref who put on the story with the "key to awake him" that everybody thinks that he is asleep, while he simply cursed himself(we dont know till now how that curse really works, but it also seems to affect his memories).
and how comes that the natsu zeref met in the past seemed to be dissapeared? conclued the link between them (acnologia-zeref) and zerefs hope "that natsu is the only one able to break him"... and what we saw about acnologia. "humans cant defeat it". did he want to say...
natsu is the only one who can stand near him and fight with him because of that muffler when other guys would die cause of his death waves?
since even gildarts cant beat acnologia.... whole FT cant defeat it with theire power combined and zeref only hopes that natsu can break him mean... that when natsu defeats him acnologia is going to return to his human form? where he mostly doesnt has the power as in dragon form.
i earlier said in this post, that zeref needed a got damn good reason to turn someone into acnologia.... i think he did that, cause he had to.... cause here he said:
http://www.mangapanda.com/fairy-tail/242/13
he had no intension to coming to this age... on the next side he thinks, that he wanted "that" to be broken... hes talking about his curse? that he can "awaken" before acnologia destroys this iland. when he had no intension on beeing there... so why the heck is he there.... cause he had to be there...
well, cause of "this" past, everything had to happen as it already happen in this manga^^.... eclipse, the door to change the world...the legend about the 12 zodiac keys can unlock a gate that changes the world... well some legends have a true core. if you can guess, the past knows about the gate because of the timetravler that appeared 400 years ago. it all comes together like a picture... cause that gate already changed the world in a heavy way. the future was so terrible, cause in that timeline... they mostly opened the gate to travel back to the past to try to stop zeref... but in the end, they were the trigger which pulled everything off.
and then the last big secrect of fairy tail: lumen etoile... i woundering if there was a fairy tail back in these days (400 years ago)... since makarov is the 3rd master... purehito the secound... mavis the first.... they all are pretty old (well mavis doesnt looks like, but shes a fairy so that should be ok) can it be that mavis already existed in that time? ivan said that lumen etoile is fairy tails darkness, mavis said its theire "light" while makarove says, its fairy tails true nature. could it be that lumen etoile is something like a "space", maybe a "prison"? to lock something pretty dangerous away? to a pretty common thought in the mythology... if a dragon dies... his soul returns to the stars. and fro some reasons, lumen etoile means nothing more than starlight.
in the time gildarts saw it, he was pretty shocked and said, what is something like that doing here beneath the guild? he mostly thinks that this "thing" in there is pretty dangerous. and since another "random" reason, in the mythology fairies are guardians....
at the beginning of the manga it said, that the dragon dissapeared....
or maybe they are all in "lumen etoile".... who knows.
its just my opinion, but i think it pretty much fits to everything so far.