If our bodies are continual..

If our cells continuously replace them selves, why do we age? Is it coded in to our DNA for systems to slow down?

If there was no disease, would we stay young?

It makes no sense to me as to why the body begins to age, our skin wrinkle and we wither away...

There is no one reason why we age. There are some popular theories though. The most popular theory is that we age do to accumulation of reactive oxygen species, or oxygen radicals, and the damage they cause to our cells. Oxygen radicals are oxygen species that have an unpaired electron, and are highly reactive. Oxygen radicals are most commonly produced through the electron transport chain. Some strong evidence for the effect of oxygen and thier role in aging exists. For example, time and time again a signifigantly reduced calorie diet in nearly all species of animals yields a signifigantly longer life span for those animals. The reason being, the less food consumed the less cycles the electron transport chain undergoes, leading to lesser oxygen radicals being produced. The opposite is also true to a lesser extent. In addition, antioxidants (which reduce oxygen radicals) have been shown to increase longevity and prevent the damage oxygen radicals cause Other theories of aging are continuous cell division leads to errors and mutations that accumulate over time, UV damage, the accumulation of damage we undertake through daily life, loss of telomere ends (which are important for cell division) from repeated cell division. Increased expression of telomerase (an enzyme important in maintaining telomeres) has increased longevity of certain lower forms of animals.