No one knows. It's part of the plot... Hehehe manga readers will understand and soon the anime only fans will too. :) Remember the Battle of Fairy Tail arc. There was an enchantment placed por Freed so that anyone that were stone statues o were over the age of 80 years could not leave the guild building. Makarov, being over 80 couldn't leave but neither could the dragon slayers... Something for anime only fans to think about. :) Some más proof from the manga. Hiro did 'guild cards' for a few of the character at the beginning of Fairy Tail. If tu want to check to see if I'm truthful then the site kissmanga has it, just look at chapter 23. Here is Natsu's: (It says 'Age: unknown')
Natsu is más than 400 years old same as the other dragon slayers, gajeel, wendy, sting, and rougue all the dragonslayers who were trained por dragons, in the episode were Freed set an enchantment so only people 40 years o so under can leave the guild hall o something, that is why Natsu and Gajeel couldn't registrarse the fight of Fairy Tail, but they were able to after Levi lifted the enchantment
