This would be an interesting statistical muse. Obviously, the vast majority of people currently die of natural causes. A few people who enter this world die horrifically due to accidents. If, theoretically, humanity conquers the aging process such that no one ever dies of natural causes, that would guarantee that 100% of people would die from some accident, eventually. Also, I suspect a world full of people who don't die of natural causes would a be a world full of wars and violence (if for no other reason, than to thin-out the population). Plus, as someone has said, who really wants to live forever in a sin-wracked world? I suspect that the tediousness of life would cause most people to go insane eventually.
So, because of the statistical certainty of everyone eventually dying from war or accident, what I believe will be a more brutish world than we now live in, and the psychological effects of living in tedium forever, I suspect that the average lifespan of an "eternal" under these conditions would probably be around 200 years.
Strange as it would seem, we strive for tedium.