No wonder creationists think that the world is not very old. How could it be, if death is not an integral part of its operation? If plants grew but never died, they would bury the Earth. If animals ate and procreated, but never died, the world would be covered with them. Elephants would be like lemmings and lemmings would be like bacteria. A world without death (which is what the Garden of Eden supposedly was) could not have been designed to last very long. To creationists, death was a curse brought on by the sin of Adam.
To scientists, however, death is the process upon which ecology operates. And evolution too. Evolution has given each organism a built-in process of senescence, which is a gradual shutting-down of the body, which ends in death. Organisms are built to grow up, reproduce, then gradually shut down and die. Death is not a mistake. It has been produced by natural selection.
If an animal lives long enough, it will eventually get killed by an accident, or will get cancer from a mutation caused by—now get this—oxygen. But now consider natural selection. Since accidents and cancer will kill old animals, natural selection does not reward mutations that confer a benefit on old animals. In fact, natural selection favors animals that take risks and pour their resources into reproduction, even though it kills them. Natural selection favors animals that do not hold back for the future, but spend their lives now on reproduction.
The evidence for this is that senescence is genetically programmed. Rather than provide the molecular details of it, let me mention some evolutionary evidence. In animals as diverse as fishes and opossums, populations that live in the presence of predators have shorter life spans than those that live in the absence of predators. If there are predators present, it is better for the opossums to have their offspring as soon as possible, for if they wait, they might be eaten. If predators are absent, the animals can wait a little longer to reproduce, grow a little bit larger first, which allows them to reproduce more. The ideal evolutionary lifespan of an animal depends on the circumstances in which its species has evolved, and is usually pretty short.
Death is not an aberration or mistake. It is not something God inflicted on the world when Eve took a bit of the apple (or, in a creationist movie filmed in southern California, the avocado). It is the product of natural selection. This does not make us any happier as individuals. But at least we can see it as part of a natural world that makes sense.