I sense that the dragon slayers are actually dragons already. Possibly dragons that wanted a peaceful coexistence with humans slowly poured their consciousness into a human form.
This is why all the dragons suddenly vanished, the transformation finally finished.
This would also explain why the dragon slayers, once enraged, are viewed as being "real" dragons by other people.
Don't forget that Natsu is older than 70(or 80?) based off of what that spell keeping him trapped.

I would also wager that sabertooth's twin dragons are weaker than Natsu simply because they didn't kill the dragons raising them, they just unknowingly killed what dragon energy had not been fully transferred to the human body. It would explain why these "more evolved" dragon slayers were ultimately inferior in power to Natsu.