| Even though you experience life as a continuous, unchanging flow of time, most of the cells in your body are constantly being renewed. Through a process called cell turnover, old cells die and are replaced by new ones, meaning much of your biological makeup is far younger than your chronological age. Aging still occurs because some cells don't regenerate, renewal slows in certain tissues, and even new cells can experience wear and tear over time. Nonetheless, on average, the cells in an adult human body are estimated to be only 7 to 10 years old — so even in middle age, much of your body is biologically closer to that of a child than to that of an elderly adult. Scientists have been able to estimate cellular ages thanks to carbon-14, a naturally occurring radioactive form of carbon that entered the atmosphere in large quantities during above-ground nuclear weapons testing in the mid-20th century. When cells divide, carbon-14 from the environment becomes permanently embedded in their DNA, effectively "dating" the moment each cell was born. By measuring the carbon-14 levels of different tissues, researchers can determine how often various parts of the body renew themselves. The pace of renewal isn't uniform across all tissues. For example, skin cells regenerate roughly every few weeks, the gut lining every few days, red blood cells about every four months, and liver cells approximately every year. Then there are cells — including most neurons in the brain's cerebral cortex and the eyes' inner lens cells — that can last an entire lifetime. Because some critical cells don't regenerate and other new cells gradually accumulate damage, the body experiences functional decline and aging, even as most cells continue to turn over. | 
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