Stem cell theory of aging
The stem cell theory of aging postulates that the aging process is the result of the inability of various types of stem cells to continue to replenish the tissues of an organism with functional differentiated cells capable of maintaining that tissue's (or organ's) original function. Damage and error accumulation in genetic material is always a problem for systems regardless of the age.
Source: Wikipedia — Stem cell theory of aging (CC BY-SA 4.0)