Okay, in my first fanfiction, I talked about how dragon slayers are actually from the past. If you didn't read it, then I'll explain it to you now, but with some extra details thanks to the manga!
So, first off, the dragon slayers were taught by their dragons, it the far far past. Then, for some reason, the dragons sent the dragon slayers into the future, with them inside the dragon slayer (read manga), just how Igneel was with Natsu.
The dragon slayer's memories were altered, thinking that their dragons disappeared on July 7, x777 and were taught by their dragons before they left, but psshhh the dragon slayers don't know that.
AND, In the Battle of Fairy Tail, both Natsu and Gajeel couldn't get past Freed's runes, and they're both dragon slayers....and over 80.