Theory on why Gajeel and Natsu couldn't pass Freed's barrier.
My own theory on this is that Dragon Slayers aren't just taught to replace their body parts with body parts of dragons, but they actually copy the body parts of their parent dragon exactly.
Naturally, since their internal organs are essentially the organs of a dragon, which allows them to consume an element dependent on the Dragon that taught them, and they copied those organs exactly from their parents, their organs are actually just as old as the dragons that taught them.
The reason I think my theory is correct, above other theories I've heard, such as Time Travel based theories, or Dragon Slayer magic being lost and ancient is incredibly simple.
For the Time Travel part, it's a simple matter of pointing out that your age doesn't change if …