I am not sure whether the time factor has been speculated on at this point, but here comes mine:
Remember when 'Eyeliner Guy' didn't age from meeting Ben when he, Ben, was a child up to and including when middle-aged Ben was his 'leader'?
Is it possible that the gift some island inhabitants get is slowed down or stopped ageing? How else could the Others have thrived in the years from the purge until now? They still had a fairly large population, so even though they couldn't replenish their number through new births, not ageing would keep said numbers up.
I don't recall that we were given any clues to the dates of the flash forwards. Jack was back at his old hospital where the staff seemed to be strangers to him.
So much seemed to have changed in what we have been led to believe is only 3 months...
I think that might explain Jack's and Hugo's misery; what good is coming back home if everyone you loved has died? Jack and Hugo might both feel they needed to get back to the island if their only living loved ones/family remain there. It would also explain the celebrity of the Oceanic 6.
I'll admit there are flaws to this (Ben did age - but as far as we know, he is the only one - even Aaron is still more of an infant than a 3 month old), but it just occured to me and I thought I'd share.