Until now, it was never explained properly in FT.
Future chapter might give a proof of the reason, or it stays a plothole.
There are some theories, like "Dragons are extinct nowadays, Natsu was raised by Igneel hundreds of years ago and made a time travel (similar for Gajeel)" or "Dragon slayers are real dragon children (with longer lifespan than humans) with some kind of transformation magic applied" (this conflicts with Zirconis´ story, but it´s still possible he didn´t know or lied. Mashima likes trolling anyways)