system
Given the following post, respond as if you're commenting on Reddit:
human
What allows animals with shorter life spans than humans (eg. Cats and dogs) to develop age related illnesses like arthritis and mental degradation in much shorter time spans?

For example a 15 year old dog may suffer from arthritis or dementia, but it would take an average human a way longer time to start developing those issues typically.
gpt
Because most age related pathology is not a specific defect like many illnesses (like MS or type 1 diabetes) but rather a general consequence of cells in your body no longer doing their job, called cell senescence.

When skin cells are no longer doing their job, your skin becomes thin, less insulating, easier to bruise and slower to heal.

When brain cells are no longer doing their job it is harder to learn new things and you are more forgetful and may even struggle to regulate your emotions.

And so on for all of the different cells in all of your different organs.

Cell senescence results in cells no longer doing their jobs, which results in both these specific pathologies like arthritis and dementia *and aging in general*.  So age related pathologies and aging in general are linked, and so you should expect age related pathologies to happen sooner in animals with shorter life spans.  As for why animals vary so much in their life span, it has to do with how much their cells are doing to repair the damage that accumulates as a side effect of normal functioning.  Cells have many ways to repair themselves and keep themselves functioning normally for a long time, and there are some cases of animal cells which seem to be able to repair themselves indefinitely.. but evolution doesn't select for longevity, it selects for passing on genes, which is different.  Evolution will select for a trait which results in more genes being passed on even if it means shorter lifespans for future generations, and some animals end up with cells that repair themselves for a long time and some animals end up with cells that don't repair themselves so well.  Dogs ended up with the latter.