If you don't agree with evolutionary theory- don't bother with this node.
What does evolution theory have to say about ageing? How can it ever be evolutionarily desirable for an organism to age? Why are tortoises so long lived? Why do cats live longer than dogs?
Ok. The normally accepted theory is as follows:
First, consider that organisms will tend to have an average lifespan due to accident, disease, famine etc. in their normal environment; in the absence of any ageing effect. People simply do get run over, they die of some contagious disease, they shoot each other; whatever it is this imposes a natural, statistical lifespan that very few individual can exceed; currently for humans it's several hundred years.
Now consider ageing, consider that most diseases and conditions of old age, such as cancer are caused by cumulative damage, genetic damage in the case of cancer; heart disease is due to blood vessel damage in many cases; strokes are similarly due to blood vessel issues. So most of the leading causes of death are directly caused by insufficient repair/maintenance over time; and all visible signs of ageing are too. This is what we believe ageing to be; lack of perfect repair.
This repair is believed to require extra energy (food) to repair and more complex genetic programming.
The theory goes that unless the necessary changes to the genes to do the repair are required by the environment, i.e. it become possible for an organism to live longer, any genes for this will not be selected for, and it will not happen.
What's the point in genes for living ten years if you usually get run over in a fortnight? These genes do you no good, and will not aid your survival; and will be lost without significant penalty due to trivial mutations. They may even do harm- doing too much repair may require more resources.
However, let us suppose that a species or part of a species suddenly becomes almost invulnerable except for ageing. What will happen? Well in a lot of cases the genetically lucky, longer lived members will end up having more children, because they have more time to do so; and their better repair processes will mean they look better for longer, and hence are more easily able to attract a mate.
Their descendents will have more of the genes that are conducive to a longer life; and hence these longer lived genes tend to spread though the population. This will keep increasing the lifespan until the species starts to die of accidents more than old age, when these genetic changes will slow and stop and a new equilibrium is reached.
Ok, is there any actual evidence for this theory?
Yes. Plenty! The best defended creatures live considerably longer than you would normally expect them to. Tortoises have excellent protection from their shell- they can live for centuries. Birds are usually able to fly away from danger- they live for decades, whereas mice which are fairly similar sizes and can't fly away from cats and predators only last for sixth months or a year. Cats are very well armed, dogs less so. Cats can live for 20 or more years, dogs usually die far younger. Humans are quite good at protecting themselves; and so we live a very long time by most animals standards.
There's even experimental support. Some worms have been bred for doubled lifespan. This was done by breeding only from the older members.
For humans, this means that, as our societies (now) contain individuals that essentially only die of old age; it is to be expected that evolution will slow down ageing over the coming few thousand generations; if nothing else were to happen.