There are many theories of aging. Oxidation reactions occur when the combustion of oxygen that keep us alive and well produces by-products called oxygen free radicals. When this process occurs in metals we call it rusting.
Free radicals are molecules that have lost an electron.
This is a highly unstable condition. To restore balance the radical either tries to steal one electron away from , or donate the remaining one, to another nearby molecule. If DNA is involved, mutations occur, a favored theory of a common cause of cancer. In time, free radical damage accumulates, thereby aging us. Free radicals are not only produced inside us, but we take them in through smoking, food, air and water pollution, x-rays, sun exposure, and various poisons to name the most common.