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At some point, you've probably heard of someone who supposedly died of old age. Usually, that just means they died of complications related to getting older, or from something a younger person might have survived, like pneumonia. Because, even though age can contribute to someone's death, it almost definitely won't be the direct cause.

The cells in our bodies do have a set lifespan, or a point at which they quit dividing, but you'll never find that written on a death certificate. It's called cellular senescence, and scientists think it's mostly because of little structures on the end of your chromosomes called telomeres. 

Telomeres are stretches of repeating nucleotides, the compounds that make up DNA, and they act as caps to protect the genetic material in your chromosomes during cellular reproduction. Each time a cell divides, those telomeres get a little shorter thanks to the mechanics of how chromosomes are copied. And, when they get short enough, senescence sets in and the cell can't duplicate itself anymore.

But, other factors also contribute to cell aging. One is oxidative stress, or damage done to your DNA by highly reactive molecules that contain oxygen. Another is glycation, which is when sugar molecules bind to key parts of your cells and prevent them from functioning.

Unfortunately, all of these things are pretty much inevitable over time. You can take some actions to combat them (like, you might've heard how antioxidants in certain foods can fight the effects of oxidation), but you can't stop them completely. Scientists aren't sure how old you'd have to be before cells just couldn't duplicate anymore, but that's because age-related diseases and injuries will catch up with someone long before they can die directly of cellular senescence.

As someone gets older and damage from all these sources pile up, their cells are more likely to have problems working correctly and fixing themselves when something goes wrong. That means their immune system gets weaker, and they become more susceptible to all kinds of injuries and diseases, like heart and blood pressure problems and Alzheimer's. When these disease appear, they can affect someone's ability to get around and take care of themselves, and can also cause problems in their organ systems. And, those will be the direct causes of death, even though age may be a contributing factor.

Thanks to Patreon patron Tommy Boyer for asking, and thanks to all of our patrons who voted on our poll to help us choose this question to be answered. If you'd like to submit your question and help SciShow keep existing, go to Patreon.com/SciShow. 

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