A biological expiration date, I think. I'm no expert, but I think I read somewhere that at a certain stage some people have been known to die simply from "old age," or something to that effect.

It's obvious that, as we grow older, our body builds up more and more "mileage" and becomes more and more worn by the passage of time and the bustle of life. Considering most of us are only fully "actualised" from our 20s-30s - a period after which we begin to slowly decline. It makes sense to me that at some point we just break down. Sure, most of the time this buildup of mileage is merely a complication and not the direct cause of death. Immortality just doesn't seem like something that is possible to achieve, as I'm sure this mileage would catch up with us eventually. Maybe life could be sustained, but would we eventually be reduced to weak, bedridden cripples?

This all sounds very pessimistic, I know, but I just think that it's the natural course of life. People are born, they live their life, and then they pass from it.