As one ages there is a loss of up to 10,000 nerve cells a day. While there is a loss of all cell types in the body as part of the aging process, nerve cells do not reproduce so the lost cells are not replaced. The loss of nerve cells results in a decreases in the function of the nervous system. The exact function lost is depended on the individual and the exact cells lost. As there are many more nerve cells than are necessary for the proper functioning of the nervous system, it is unlikely that routine loss of nerve cells causes any apparent problems until advanced old age.
Some apparent decreases in functioning include decline in speed of response and ability to integrate what is observed than there is in verbal ability or memory. There is also an age-related decline in intelligence, as measured with standard intelligence tests. Memory is also affected by age.