One of the theory I always been entertaining is that most the the Losties may have been the kids we see living with the others. They have the right age for it and we are never explaIned what happened to the kids when Ben became adult. These kids may have no conscious memory of their time on the island. Other older Losties may have been there too as adults.
Maybe the plane did not crash but was staged. When people breathed through the masks, they lost consciousness and later were put in the wreckage among other victims. That's why they had superficial wounds.
This theory would explain several things (not everything):
1- The impression that they knew each other before the crash. Kate said to Charlie that he looked familiar. Jack and Kate also instinctively got close.
2- Bernard might have been on the island previously and wanted Rose to be brought back there so her cancer would be cured.
3- Hurley's father was gone for 17 years when he got back in his life. Was he on the island? Did he arrange for Hurley to be brought there?
4- Who was Jin father? The guy in the Dharma video?
5- Jack sees his father on the island several times. Is he recalling memories of his youth but distorting them as his father of the present? And since claire is his sister, she might have been brought up on the island too. Is it why she could deliver her baby? Because she is second generation islander?
It would make sense that they were on the island as children.