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

What exactly causes any living thing to cease living simply from old age? Why can't cells perpetually replace themselves and continue to grow and live?
gpt
Once the germline is established an organism begins to die, due to the loss of cellular quality control. After you have offspring, you are no longer important (evolutionarily speaking). 

Your genome is also limited in the number of times it is copied, due to sequences known as telomeres. These get shorter and shorter with subsequent divisions and the grnome will begin to deteriorate after further replication. Telomerase is an enzyme which repairs these regions but it is not expressed after a certain stage of embryonic development. 

Both of these processes are often reactivated in cancer cells, allowing them to grow out of control.