The limited capacity of all normal human and other animal cells to reproduce and function. The gradual decline in normal physiological function of the cells is referred to as aging or senescence. The aging process ends with the death of individual cells and then, generally, the whole animal. Aging occurs in all animals, except those that do not reach a fixed body size such as some tortoises and sharks, sturgeon, and several other kinds of fishes. These animals die as the result of accidents or disease, but losses in normal physiological function do not seem to occur. Examples of cells that do not age are those composing the germ plasm (sex cells) and many kinds of cancer cells. These cells are presumed to be immortal.