system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes animals to die of old age?

Like why would an otherwise healthy being just die all the sudden? Is death of “natural causes” an actual thing or is it a catchall term for more specific things that just tend to happen as animals get older?
gpt
The cells in your body are constantly replicating themselves and then dying off.  Your hair and nails are made of these dead cells (and that's why your hair and nails grow).

However, every time a cell replicates, it's not exactly perfect.  There's a piece of the cell that acts almost like a timer.  Every time the cell replicates, that timer piece gets a tiny, tiny bit shorter.

Have you ever photocopied something?  You know that the photocopy isn't quite the same?  There's always a slight be a degradation.

Well, that's what happens with a cell.  So eventually, the degradation becomes so bad that the cell can't perform it's function correctly.  That's why we get symptoms of old age as well.

Then eventually, the replicated cells just start to fail and that eventually leads to death.