A couple of facts and questions.
– We know that the Purge occured on December 19th, 1992.
– We know that before the Purge, DHARMA were experimenting with Polar Bears on the island.
– We know that some Polar Bears survived the Purge.
– Do we how old the experimented Polar Bears were?
– Do we know when the Polar Bears were brought to the island?
Because I did a little research on Polar Bears, which said that they live from 15-18 years in the wild. So even if the ones we see were brought to the island as infants in 1992, they would be around 12-13 years old in the wild (and not their normal habitat) AND have survived an island-wide toxic gas, and I assume they were not given gas masks. And this is the best case scenario. I doubt DHARMA would experiment on baby bears, so assuming a bear would be at least 8 in 1992, that would make the bears over 20, living in the wild, surviving toxic gas.
So this could be something that the writers have sort of looked past, saying “who cares, it’s just the bears”, or maybe the islands aging phenomenon we sometimes see extends to bears as well, which is why we keep seeing them (unless we constantly see them to make more “problems” for the losties). Also, there are so many references to polar bears, like Hurley’s magazine, Michael giving Walt a stuffed Polar Bear a a child, a polar bear in the Drive Shaft commercial crib etc.
Just some food for thought.