Even if there were a hard-coded expiration mechanism, that mechanism would presumably have limited precision and a non-zero failure rate (assuming it's implemented with biochemistry rather than magic). Looking at the list I linked to, it does rather look as if there were a cunning device to kill people towards the end of their 118th year, and that device failed in two cases... but I can't imagine why there should be such a device or how it might be implemented, so I don't really believe that.
Even if the chances of surviving another year did stabilise around 50% after the age of 100, say, you still wouldn't have enough people living beyond 130 for you to be able to measure with reasonable accuracy the mortality rate of people aged over 130. So the hypothesis that a person aged 140 would still have a 50% chance of surviving another year would be totally untestable.
If you think about it, there's also no hard limit for how tall or how short an adult human could be. Except the limit of zero for the height. Again, beyond a certain point there will be, in practice, no data available for testing any hypothesis about the distribution.
