The pieces and parts that make up your body update at different paces. In a race, your stomach lining wins, being replaced every two to nine days; while the lens of your eye and most of your brain are the same as when you were born.
NPR's science blog Skunk Bear team produced a handy 3-minute video, "How Old Is Your Body, Really?" hosted by reporter Adam Cole to help explain how these processes take place:
"Certain parts of your brain add a few new neurons over the course of your life," Cole explains, "but the vast majority of your neurons developed before you were born. It's the connections between those neurons -- the circuits that store memories -- that are constantly changing."
In order of update:
2-9 days: Stomach lining – your gut epithelium regenerates relatively quickly because of its exposure to stomach acid and bile.
7 days: Taste buds – the taste buds have about 10 to 50 sensory cells each that are renewed every week.
2-3 weeks: Lungs
2-4 weeks: Outer layer of skin (epidermis)
4 months: Red blood cells – 100 million cells are born every minute, while 100 million others are destroyed.
6 months: Nails
2-7 years: Hair – 100 hairs fall out every day and are generally replaced.
10 years: Skeleton
15 years: Muscle
25 years: Fat cells – they may seem to multiply, but they actually expand and contract.
Life: The lens of your eye, most of your brain and about half of your heart, stay with you from birth to death.
Scientists have been able to track cellular regeneration, in part, because of nuclear bomb testing of 1945 and 1963, the Skunk Bear team revealed on its blog.