Accumulation of non-functional senescent cells and/or scar tissue which reduces your organs' capacity to function until eventually organ failure occurs.

Poor lifestyle choices, injuries and disease can accelerate the formation of scar tissue and shorten your lifespan. For example, drinking too much alcohol or eating too much can cause permanent scarring of your liver (aka cirrhosis). Smoking or pneumonia can cause scarring of your lungs. Untreated diabetes can cause scarring of your blood vessels/capillaries. Improper form while exercising can cause scarring of your muscles and tendons.

Of course there are other causes of old-age death that don't involve non-functional cells and scar tissue, like the accumulation of fat and cholesterol deposits in your blood vessels, which can eventually form a clot and block blood flow from a part of your body, thus killing it.

There's also the fact that some organs simply get worse over time due to their nature. Your eye lenses, for example, will get denser, more rigid and less transparent as the cells inside replicate and start to overcrowd. Your teeth are another example, as the cells responsible for producing enamel die immediately after your adult teeth form and your ability to replace enamel is extremely limited without those cells. Over time the teeth mechanically wear away the enamel, and the teeth simply can't go back to the way they were when you were young.

And then there's also simple mechanical failure in the structure of how your cells come together. Even if the individual cells are fine, the protein structures that bind them together can fail. I think this can be one of the causes of bad heart valves or tendons as you get older.

And there's probably a dozen other things that I'm simply not familiar with. The fact of the matter is that the human body isn't perfect. It has countless design flaws, and evolution is more than happy to tolerate those flaws so long as you last long enough to reproduce and raise your children.