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 by Freed so that anyone that were stone statues or 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 more proof from the manga. Hiro did 'guild cards' for a few of the character at the beginning of Fairy Tail. If you 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')
posted over a year ago
I thought the over 80 thing was just a joke. Hard to tell with FT....go Natsu!
He isn't E.N.D.- exactly. E.N.D. stood for Etherious Natsu Dragneel so I think it's more like a version of Natsu but not Natsu himself. More like a prophecy he was going to exist and this is something to control him kind of a thing. Like Zeref could see the future or something.