An article of quotes from various noted aging reseachers: "Aging is caused by the gradual, lifelong accumulation of a wide variety of molecular and cellular damage ... The free radical theory is the most widely accepted theory of aging. But the idea that aging is caused by one thing is naive. One general theory can never fit all. Clearly, it's the combination of genes that your parents dealt you and the lifestyle choices you make and the environmental toxins one is exposed to. One need only count the number of ways a car will fail to start to appreciate that aging can be caused by a large number of problems. Like any machine, it's going to wear out ... about 25 percent of how a person ages is due to inherited genes. Certain genes control a cell's ability to repair damaged DNA. If those genes are defective, they can't do their job. ... Not everybody will be susceptible to diseases like Parkinson's or cancer as they age. But each one of us will lose muscle mass and muscle strength. That's why this research is so important. Frailty affects all of us."