So anyway, in the latest chapter, it was revealed that:
Natsu and the 4 other Dragon Slayers from 400 years ago actually met some time before they were sent to the future. Quoting Zeref, most likely because of the dragons in their bodies making this stuff (I forgot the name) to prevent them from turning into dragons themselves, they have mostly forgotten about meeting.
Before they time travelled, they all seemed the same age. And Lucy's mother opened the gateway (I assume) just once for them all. She didn't have all keys and became ill until she died, so I think we can definitely assume she didn't do it again.
Then how is it that Sting & Rogue are generally the same age as Natsu in the past, yet have to catch up to Natsu, Gajeel and Wendy who are frozen (petrified, Fairy Sphere'd) for 7 years?
I mean, it could be a reason we don't know yet. Or am I just missing something VERY simple?