There are two theories as to why people age. One theory posits that aging is naturally programmed into the body. The second theory is that aging is the result of damage that has accumulated during life, which is why some people live longer than others.
Everyone ages differently, and scientists believe that the rate at which someone ages comes down to four main things: chemistry, behavior, physiology and genetics. Studies have proven that when scientists adjust the genes in yeast cells, mice and other organisms, the life span of those creatures can be nearly doubled. It is believed that genetics makes up about 35 percent of a person's aging process.
Even with good genes, the body continues to undergo chemical changes throughout its life. Some of these changes cause damage to the body and are factors in the aging process. DNA repair, hormones, heat shock proteins and free radicals are all important concepts when looking at the biochemistry of aging.
As a person ages, certain organs in the body change. This includes the heart, which thickens over time. This thickening is due to a thickening of arteries. This gives the heart a much lower maximum pumping rate. As the immune system ages, it takes longer to replenish T cells. As a result, older people take longer to get over being sick.