They've stated that they're planning to train their astronauts in dentistry, medical technologies, etc. They'll have years of full-time training before they go, which should be enough time to get a decent grasp on the basics of many different fields. Certainly complicated procedures will be out of the question, but basic health care should be within reach. A toothache is not going to put them out of commission. Too, the company will undoubtedly select particularly healthy individuals and carefully control their diet and exercise. They'll also remove a lot of modern dangers from the equation. No sports injuries, violent crime, car crashes, pollution, disease, etc. Seems to me there's a good chance that, assuming their equipment remained functional, they'd live until advanced age caused an untreatable health condition. At which point they'd die a relatively natural death at a reasonable age, which would be acceptable to most people considering the trip.