Somewhere i read a theory which seemed to be quite logical:
The whole "alternative universe" thing is taking place in the future, after events which we are seeing now. Maybe Jack or some other character will somehow move the island where it cannot crash the plane. That would explain why Jack looks a bit older.
The basic idea is the we just saw the actual ending of lost, those flashes we saw will turn out to be ending of lost and everything else was just a progress.
Maybe that's why Juliet talked nonsense when she was dying, her consciousness traveled to the future like Charllotes traveled to the past when she was talking about chocolate, and Juliet wasn't refering to the jughead's "it worked"
anyway,
I really like that idea. and now I feel a bit spoiled.