Aging is a highly individualized matter. Aging proceeds at different rates in different people and at various rates in various systems of the same person. There are several theories that account for the physical changes coming with age, but none explain adequately why these changes take place.
Some theories emphasize hereditary factors and loss of cellular mass.
Some cells, as skin and blood, do not divide and reduplicate. Other cells, as brain, nerve, and muscles, do not divide and once they are gone, are lost forever. Another alteration in aging is the change in connective tissues which bind the body together. With age the connective tissues harden without sufficient lubrication. The fibers stick together and flexibility is lost.