These are questions that interest everybody, as attested by the plump profits of beauty-products companies.
Evolution has several explanations for our mortality.
Such organisms are, in effect, immortal. For the rest of us, it is possible to argue that once we have done our bit for reproduction, our bodies are expendable and will slowly degenerate until we die. There are three interrelated genetic theories to explain ageing and death. The second idea, related to the first, is that genes with deleterious effects in later life are maintained in the population because of their greater benefits in youth. This 'trade-off' between profit and loss is related to the third theory, which acknowledges that resources are scarce, and must be apportioned between survival and reproduction.