I was listening to an interview with some researchers on super seniors, generally described as those who get to ripe old ages, but are still lucid and active. Anyway, someone offered a fact, which one of the researchers quickly supported. From memory it is something like, "You have better chance of making it to 80 than you do to 70."
I have a vague intuitive sense that this is right, but I'm not sure how such a study was conducted. Perhaps the percentage of those in their 70s who make it to 80 is higher than the percentage of those in their 60s who make it to 70. Of course, I don't have the data. I'm just trying to find a way that may explain the claim.