Ok, now you all remember the barrier that Freed put up during the Fighting Festival Arc and how Natsu and Gajeel couldn't get through? It was bothering everyone a lot for a long time. A lot of people figured Gajeel and Natsu were older and the dragons performed a spell to slow time down on them? I think the reason they couldn't get through was because the dragons were inside of them! Think about it, Gajeel and Natsu's dads were 400+ years old. So because of their ages, Natsu and Gajeel couldn't get through the barrier. It makes sense! They were technically of age but they're foster fathers weren't!