Red blood cells live for about four months, while white blood cells live on average more than a year. Skin cells live about two or three weeks. Colon cells have it rough: They die off after about four days. Sperm cells have a life span of only about three days, while brain cells typically last an entire lifetime (neurons in the cerebral cortex, for example, are not replaced when they die).
There's nothing special or significant about a seven-year cycle, since cells are dying and being replaced all the time. It's not clear where this myth began; perhaps some well-meaning but innumerate person simply added up the all the lifespans of the body's various types of cells and (mistakenly) assumed that all the cells are renewed after seven years.
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