So, when the bomb went off... that created the "Afterlife" or whatever you want to call it. The bomb created it because, as someone told me in a post, they each wanted to detonate the bomb because they wanted to go to a time when the plane never crashed on the island. In the Afterlife, that is what they got. The plane never crashed because the Island was underwater. The Losties could only enter the Afterlife when they died. Since there is no time in the Afterlife, it did not matter when each of the characters died. For example, Jack died at the end of the episode, The End, but Kate, Sawyer, Claire, etc. did not. They could have lived to be 100 years old and entered the Afterlife decades after Jack and others had already entered it. Time could move so slowly in the Afterlife that it seemed like a nano second went by but in the real world it was many years because time does not exist in the Afterlife so none of the early dead people noticed any time passing for the ones who died later. Once all of the Losties realized their time on the island and found their soul mate, they were able to move on to a higher place.