(NB: The danger of writing in advance, is that someone gets in first! Oh well. Please see this much better essay on this subject by Karl D Stephan on this site.) One of the reasons that many countries’ populations are still growing despite below replacement birth rates is that, thanks to better diets, lifestyles and medical advances, people are living longer. Thus, to use a metaphor, if a country’s population was to stand on a train station platform sorted by age, there are fewer babies being born onto the platform, but the platform has been getting longer, meaning that the elderly are not leaving the platform through death at the same rates as they have in the past (I’ve been watching Thomas the Tank Engine with my sons, the dulcet tones of Ringo Starr are still in my head…)
Now I always assumed that this would mean that, not only are more people living longer, but also that the upper limit that we could hope to live for is also going up. Apparently, according to researchers for the Albert Einstein College of Medicine, this was true until the 1990s, but since then our upper limit of lifespan has hit a plateau and the longest life humans can expect to live for is about 115 years. Although some, like Jeanne Louise Calment from France who was 122 when she died, will live longer, on average the most we can hope for is 115 years. This levelling-off of our maximum lifespan is new: for centuries both our life expectancy and our maximum lifespan has been rising. As the leader of the study said:

“Demographers said that if there’s an end in sight, they couldn’t see it ... When Calment died at 122, everyone said it’ll only be a matter of time before we have someone who’s 125 or 130. But after Calment, there was no one else. It was 115 … 115 … 115.” The study focussed on France, Japan, the UK and the US (the countries with the most supercentenarians – those who are 100 or older) and worked out the oldest individuals who died in any given year. Between the 1970s and 1990s the maximum age rose from about 110 to 115 and then around 1995 it stopped rising. According to Jan Vijg, the study's lead author, although sanitation, antibiotics, vaccines etc have advanced during that time, there seems to be a ceiling hardwired into our biology. As we get older, our DNA and other molecules get damaged which leads to diseases like cancer or Alzheimer’s. Interestingly though, if you live past your 80s and 90s, your odds of getting such a disease actually declines. Indeed, supercentenarians don't tend to die of major diseases and many are physically independent even at the very end of their lives. But they still die, “simply because to many of their bodily functions fail” says Vijg.
So while we have been getting better at getting more people to older ages and to keep them healthier for longer, the study concludes that there is a “ceiling” beyond which we do not tend to live. Now, this study may simply have picked on a couple of decades which are anomalous and the maximum age at which we can expect to die may continue to rise in the foreseeable future. But even if it does rise, can we expect people to live until they are 130? 150? 200? Is there a limit, if not at 115, but at some point? Surely our body just breaks down at some point doesn't it? And quite frankly that seems to be a good thing. Overlong life, or immortality, seems like an awful prospect.