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.
Thanks. Much of what we call aging is oxidative processes. A gene that once encoded a protein properly no longer does, and the misfolded protein doesn't do as good as a job. That actually happens in your skin, and it plays a role in why old people have less elastic skin. However, we could in theory swap out the bad copies and replace them with new ones. Your skin would once again make the correct protein, and your skin would grow youthful looking again. All of what we call aging comes down to physical changes. And these physical changes can, in theory, be reset. If your skin is new, and your bones and muscles strong, and your vascular system healthy, you've started to really turn back the clock. Our cells break down after a number of divisions because it wasn't important for our evolution for them to go on forever. However, single-cell species don't die off because they get older with each new division. If you could imagine each cell in your body to be as viable as a single-celled organism, you'd have a body that didn't age very much.