I have thought that what if all the dragon slayers are directly related by blood and that either mavis or zeref could be the parent… at a time zeref was bad but wanted to know the value of human life as what the latest chapter said… he knows mavis is powerful and is capable of killing is already dead so looks for natsu instead acknologia knowing this attacks fairy island… I strongly believe that acknologia is really the bad guy here and not zeref.. it's not like zeref commanding acknologia in fact he seem to fear was surprised when acknologia appeared and that mavis could be a friend of the 3 dragons and entrusted his sons and daughter to them. in the hope that one day their combined powers will be able to defeat this dark dragon.Actually, I think it would. Even if they're 400 years from the past that would still technically make them over the age of 80 in their time if you consider their birthdate. Their bodies may not be physically aged past 80 but it would technically make them over that age.
I don't believe that them having time-traveled from the past being possible because that would make the barrier reject them. Even if they had traveled 400 years into the future that wouldn't change their actual age.Those children were given to the care of the "good" dragons so that they would be trained as dragonslayers and either by fate or some plan behind their growing up they would end up in Fairy Tail. That would explain their age being more than 80 years old. Also we saw that Natsu's scarf protected him from Zeref's death magic, which means that Igneel gave it to him, expecting him to run into Zeref. Since I love reading books, especially chic lit, I love reading shojou manga as well. What I love most about reading shojou manga is the fact that it's all still pictures. It's like you freeze frame every sweet moment, the revelation that something horrible is about to happen and that there is no escape.