Does your body really replace itself every seven years?

If your body really replaces itself every seven years, does that mean everyone in this picture is the same age internally? See pictures of healthy aging.
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Know yourself; love yourself; be true to yourself. These old adages have been batted around throughout the years by a whole cross section of artists, entertainers and philosophical types, from John Paul Sartre ("We only become what we are by the radical and deep-seated refusal of that which others have made of us") to Bob Dylan ("If you try to be anyone but yourself, you will fail") to Katharine Hepburn ("If you always do what interests you at least one person is pleased").

But how do you get to know yourself when you are constantly changing? Whether it's shedding skin, renewing the lungs or growing new hairs, the human body is in constant flux.

According to researchers, the body replaces itself with a largely new set of cells every seven years to 10 years, and some of our most important parts are revamped even more rapidly [sources: Stanford University, Northrup].

Some of you may be thinking, "Well, that explains why my spouse/sibling/parent/co-worker acts like a little kid." Others might be expecting those new cells to be the key to a longer life. Unfortunately, it's a little more complicated than that.