I'm reading a book about this: "Long for This World: The Strange Science of Immortality".
There are many contributing factors as to why we get old and die. But, basically, I believe it happens very gradually on a cellular level. As we get older, our cells become less and less able to clean and rebuild themselves. Garbage builds up and doesn't get cleaned out. Dead cells sit there and don't get replaced. Crosslinking stiffens us and makes us creaky. Our arteries and brains get clogged with gunk.
Many scientists are now working on solving some of these problems - how to clean out the debris. It may turn out to be a microorganism that's needed to eat the gunk but it won't be easy because it needs to be specific and not eat everything in sight.
At any rate, it's an interesting read and I may add more to this thread as time goes on.
Feel free to add any thoughts you may have on this subject.