“Why we age” is no longer a solely philosophical question. In parallel with the rising
awareness of the social ramifications of an aging population, basic research has expanded our understanding of the intricate nature of biological aging. The present paper aims at discussing our current understanding of the molecular and cellular alterations that accompany aging. To this end, the main theories on the mechanisms of aging, error theories and program theories, will be discussed. Special focus on neuronal aging is also presented to provide illustrative examples of these aging mechanisms.