Everyone on the island seem to never age. Evenmoreso, some seem to have been physically cured from any ailments they have had prior to arriving on the island: Locke's paralysis, Rose's brain cancer, Charlie's drug addiction, even Ben's back tumor (and by the way, how did he heal and was up & going so quickly?). But in the same token, some seem to not get cured, but rather get hurt and/or die: Shannon, Ana Lucia, Boone, Mr. Eko, Libby, etc. Furthermore, maybe that's why experiments take place on this island, to test the fountain of youth's theory and how it can be a possibility. This is also why kids are of big interest to the Others, because of their youthfulness. What better subjects can a fountain of youth situation have but...youth. And as with any experiment/s, to succeed you have to fail first. Also, things can and will go wrong, in this case on the island, they have. And as far as all of the "survivors" being connected to one another, well, that's another mystery in itself. Remember, all this....just a theory.