system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do different species have different life spans? For example we live to a maximum length of years, yet a giant tortoise can live to like 170 years old? What determines this?
gpt
Genetic programming, basically.

First, take all other causes of death - accident, predation, suicide, and so on - out of the picture so you're only dealing with creatures dying "of old age".

What happens as most creatures age (there are some exceptions) is their cells eventually can only replicate so often and repair or replace components within their body so many times. Eventually more and more of those cells run out of times that they can duplicate and replace themselves. They die, the organ or body part their in dies or becomes critically damaged when enough cells within them die, and of course so does the organism.

Different animals duplicate their cells at different rates, and there's a decent correlation between size of the creature and length of its natural lifespan. Mice grow much faster (and breed much faster) than men, and run through their cells' "max refresh count" a lot faster as a result. So they live a shorter life. Galapagos tortoises run their counter a lot slower, however, and so they can live a lot longer.

It's also correlated to how fast the animal goes (hummingbirds live a lot fewer years than giant pandas), but not perfectly so.