User blog comment:FairyRaven/Fairy Tail Chapter 301: The Dragon King, Review and Commentary/@comment-5170433-20120930014845/@comment-5268982-20120930083610

I admit that it's one of the most well-thought theories I've seen. It actually makes perfect sense. I too begin thinking that the Dragon Slayers may have existed in the past, along with their Dragons, or that the Dragons that trained Dragon Slayers were simple survivors.

However, for 2 reasons, I believe that the Dragons indeed existed 7 years prior to the show: That's why I believe Igneel, Metalicana, Grandeeney e.t.c survived to the present and are not ancient Dragons who put their Dragon Slayers into sleep for many many years. I may be wrong though.
 * It is said that they disappeared in X777, and the show began in x784.
 * Daphne filler arc. Sure, it's a filler, but the plot is probably given by Mashima himself. In that, Daphne actually saw a Dragon with her very eyes, which means that not all Dragons are ancient.