The build up of senescent cells is one of the contributing causes of aging, and is partially due to the progressive failure of the immune system to destroy these cells as they crop up. Many of the changes that come with aging accelerate as they progress, and this piece provides one example as to why this is the case; for senescent cells, the more you have the faster they accumulate: "Cells may become senescent in an effort to protect the body such as when tumor suppressor genes shut down division to prevent cancer. However other sorts of damage may lead cells to stop dividing as well. A pivotal study last year showed elegantly using a trangenic approach that if senescent cells were regularly cleared from the body of mice, signs of aging in many tissues were dramatically reduced. The explanation for this result was that somehow senescent cells were damaging nearby cells, perhaps by excreting toxic materials. ... A newly published study [proves] or the first time that senescent cells do indeed damage nearby cells causing them to become senescent too. It also shows this occurs through direct cell to cell contact and resultant spread of reactive oxygen species. Furthermore it shows evidence this process occurs in the living organism as clusters of cells bearing senescent makers are found in mice livers. Clearly the next and important step for helping to reduce aging in humans is developing a safe and effective method presumably using a pharmacological agent in which senescent cells can be removed from the body."

Link: http://extremelongevity.net/2012/06/11/study-proves-senescent-cells-cause-nearby-cells-to-become-senescent/