How does a tortoise live so long, like 150 yrs, etc.?

Many animals do not die of old age. They will live, and grow, until something kills them — be it starvation, predation, disease, or misadventure. Some reptiles are like that. read more

So, basically, some animals live long lives because evolution has selected them to live long lives - we all have an expiration date in our genes, and the reason has to do with maintaining the ability to evolve. read more

The giant tortoise has an armored shell for protection, but it also has a couple of other features that correlate with long life. As an island species, it has enjoyed natural protection from predators over the course of its history. It's also very large, and larger animals tend to live longer than small ones. read more