The changes that occur in an organism (or a part of an organism) between maturity and death, i.e. ageing. Characteristically there is a deterioration in functioning as the cells become less efficient in maintaining and replacing vital cell components. In animals this results in a decline in physical ability and, in humans, there is also often a reduction in mental ability. Not all the parts of the body necessarily become senescent at the same time or age at the same rate. For example, in deciduous trees the shedding of senescent leaves in the autumn is a normal physiological process. Various theories have been advanced to explain why ageing occurs. The telomere theory of ageing stems from the finding that the tips (telomeres) of chromosomes in body cells shorten with each round of chromosomal replication and cell division. It is suggested that eventually essential genetic information at the chromosomal extremities is lost, thus disrupting vital cellular functions and causing cell death. An alternative account is the mitochondrial theory of ageing, which proposes that over an individual’s lifetime, mutations accumulate in mitochondrial DNA to the extent that there is an age-related decline in the formation of ATP by oxidative phosphorylation. More recently it has been proposed that changes in the epigenetic modifications of DNA, and of the histone proteins that package DNA in chromosomes, could be linked to age-related functional deficits (see chromatin remodelling). For example, there is a gradual loss of DNA methylation with increasing age, which might disrupt patterns of gene expression. Similar changes are associated with certain cancers. Other proposed mechanisms of ageing in humans include decline in immunological function, cross-linking of proteins or attack by free radicals, both of which lead to tissue damage, or simply the wear and tear of components (rather like an old car).

http://www.senescence.info/ Examination of the causes and consequences of ageing, compiled by João Pedro de Magalhães of Harvard University