In episode 504 "The Little Prince" we see Sawyer, John, Juliet, Miles, Dan, and Charlotte in the same timeline as when Oceanic 815 first crashed on the Island. We see Claire giving birth and the Hatch lighting up when John was pounding on it. I think that this proves that the idea of the same person existing in one timeline at two ages is not "not allowed" by the rules of time travel. Locke was in a reasonable vicinity of himself, as was Sawyer and Juliet. They could have interfered with their actions from that night. Obviously they didn't and the deterministic theories about time will say that they could not interfere and that's why it's not a paradox. Perhaps, but you can't have your cake and eat it too. The reason why Sun and Ben are in the present and why the rest of the Oceanic 6 are in 1977 can't have to do with this idea that a person can't exist in the same time in two forms. We've already seen it! happen. It could be argued that Jin and young Ben do certain things in this period of time that would alter the timeline catastrophically if older Ben and Sun were to come to that time period. That's a different point, though, and up for speculation. Let's not cling to theories that the show has already disproven, though. Theory by Polonius
