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 your time travel. You're still only as old as you were when you left your home time. I will admit however that, depending on view points, some Time Travel theories could work.
As for the Dragon Slayer magic being lost and ancient, well...
Other magic also exists. It's simple, and sounds pretty stupid, I know, but let's face it; the magic the other characters use can't all have been created within the last 80 years. Especially Lucy's Celestial Spirit magic.
Because of this, it wouldn't make sense for Natsu and Gajeel to be the only ones trapped if Freed's barrier also stopped people who used magic that was over 80 years old.
I know not everyone will agree with my theory, but tell me what you all think of it. I'd like to know what others think of my theory.