I've been thinking long and hard about this concept lately. I'm getting old(er), and I'm noticing that I'm starting to slow down. I've still got 20, maybe 30 years of good life left, but really I don't see the point of living much beyond my 60s.
Logan's Run had the right idea. People increasingly just "get in the way" of progress at a certain age. It does vary for some of us, but I'm already seeing that in some ways I'm holding society back by extending my life. The next generation is more tolerant, more enlightened, and certainly more technically competent than I could ever hope to be.