I work at a VA hospital, so I get a lot of these folks. It's just like with any other patient, you have to find a reason that's appealing to the patient, and approach from that angle. Cosmetics, function, and health are all still strong motivations for a lot of the older folks, and a lot of times when they give you the "I'm just gonna die anyway" line, all they're really doing is asking for a little bit of reassurance from you that it'll be worthwhile to have the treatment performed.
Just as a recent personal example, I have one particular patient who's wheelchair-bound w/severe COPD, for whom I just finished a #8-10 Captek FPD. When I recommended the bridge, he asked why he should bother; I just briefly explained that fixed prostheses are stronger, more cosmetic and more effective, and joked that since everything else in his body was obviously so durable, I wanted to make sure he had teeth to match. After a couple seconds' thought, he agreed and we scheduled him for the prep. When I delivered the bridge, he was very happy with the way it looked and functioned, appreciative that we gave him a permanent tooth instead of a removeable one, and suddenly his age didn't seem to matter as much as it did before we started.
In the end, I've had the greatest success when I approach them with a realistic, but still optimistic and encouraging, attitude. Some folks just aren't interested in saving their teeth or maintaining their oral health, but you get those patients at any age. With the older/sicker patients, you just want to make them feel legitimized and validated, and present them with an end result they can look forward to achieving.
