tumor != cancer (Score 1)

Actually the tumors that most female rats acquire with age (they're not nearly as common in the males) are benign tumors of their mammary glands. It's not cancer, where the tumor cells mutate to spread throughout the body. But, if not removed, these tumors will eventually outgrow the rat itself, making it nearly impossible for the animal to move around to get food and water, and it will starve or die of dehydration -- not cancer.

That's just one of countless examples of how nature could not care less what happens to organisms after the typical period of sexual maturity and reproduction. The genetic code is optimized for vitality in the reproductive stage, and I think those same optimizations lead to all manners of gruesome and painful death on the other side of the hill.