User blog comment:Sargenttacos115/Natsu and Gajeel age Theory/@comment-121.54.8.130-20121203051353

in X777 Natsu must be 10, Wendy is 5, and Sting, Rogue around 4 years old.. in X784, Natsu past 17, wendy 12 and Sting, Rogue around 11 years old.. in X791, Natsu still looked like 17 or 18, Wendy stil look 12, only Sting and Rogue grew older, around the age of 18. Gajeel still looks like early 20s.

so it's possible that all of them came from the past (400 years ago). they are kids back then and learning dragon slayer magic. But because of Acnologia, they were sent to the future to survive. Their foster parents went into hiding and the kids sent to the future to protect them.

That's my theory..