Aging

Just How Old Are You?

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How old are you? Well, that all depends. Are you the same person you were 7 years ago? You are, in fact: no one knows where the idea that your body replaces itself every 7 years came from, but even if it was a rough average of every cell's lifespan, it would still be wrong. Some parts of you remain for your entire life. The cells on the inner lens of your eye have been there since you were a fetus. Your tooth enamel is never replaced, and neither are the neurons in your cerebral cortex. Luckily, neurons in other parts of your brain do regrow. As do other parts of your body: colon cells, for instance, refresh after only four days, and you get an entirely new skin every 2-3 weeks. White blood cells take more than a year to regenerate, and it takes an average of 10 years to grow an entirely new skeleton.

You Have More Than One Age

Scientists know how old each of your cells are, and they have the Cold War to thank.

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Key Facts In This Video

  1. The outer layer of your skin regenerates itself every 2 weeks. 00:28

  2. Scientist can date your organs by measuring the amount of carbon-14 in them. 02:01

  3. One study found that in a 90 year old brain, the neurons in the hippocampus were only 20-30 year old. 02:50

Do You Really Get A Whole New Body Every 7 Years?

It's not true that your body completely replaces itself every 7 years -- or ever.

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Key Facts In This Video

  1. Your body mostly replaces itself every 7–15 years. Some parts are never replaced, and some parts are replaced much more quickly. 00:19

  2. The organs that work the hardest have the fastest changeover. You get a whole new skin every 2-4 weeks, red blood cells last only half the year, and your liver renews itself at least once every couple years. 00:37

  3. Some parts of your body stay with you for life. The cells on the inner lens of your eye were formed when you were an embryo. Tooth enamel never regrows, and evidence indicates that you can't regrow the neurons of the cerebral cortex. 01:14

Why Does Time Speed Up As We Get Older?

All sorts of things can affect your perception of time.

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Key Facts In This Video

  1. When we're focused on something, we don't notice the time passing. That makes it seem shorter than it actually is. 03:10

  2. Our sense of how long something is correlates with how much energy our brains are using on it. Brain energy use peaks around age 5, which could be why time seems to speed up as we age. 04:14

  3. Being afraid and being bored both increase our perception of time. 04:50

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