The body renews itself at varying paces. Just how long the cells in certain areas last depends on how much work they're asked to do. Red blood cells, for example, enjoy a quick life span of only about four months as a result of their arduous journey through the circulatory system, carting oxygen to tissues throughout the body [source: Wade].
Skin: The epidermis sees a fair amount of wear and tear, thanks to its role as the body's outermost layer of protection. These skin cells rejuvenate every two to four weeks.
Hair: The body's natural fuzz has a life span of about six years for women and three years for men.
Liver: The liver is the human body's detoxifier, purifying a wide variety of contaminants from our systems. It's aided in the process by a constant blood supply and remains largely immune to damage from these toxins by renewing itself with new cells every 150 to 500 days.
Stomach and Intestines: Cells that line the surface of the stomach and intestines have a difficult, short life. Constantly battered by corrosives like stomach acids, they typically last only up to five days.
Bones: Cells in the skeletal system regenerate almost constantly, but the complete process takes a full 10 years. The renewal process slows down as we age, so our bones get thinner.
Despite all this regeneration all the time, people who want to live forever shouldn't give up on that search for the fountain of youth. The truth is that we still get old and we still die. Frisen and others believe that this may be because of DNA mutations, which worsen as they're passed along to new cells over time [sources: Wade, Epstein].
There are also some cells that never leave us and may aid the aging process, or at least the body's breakdown over time. While the eye's cornea can regenerate itself in as little as one day, the lens and other areas don't change. Similarly, neurons in the cerebral cortex – the brain's outside layer that governs memory, thought, language, attention and consciousness – stay with us from birth to death. Because they aren't replaced, the loss of these cells over time can cause maladies like dementia. The good news is that other areas of the brain, like the olfactory bulb that helps us smell and the hippocampus that helps us learn, can and do rejuvenate [sources: Wade, Epstein].
So get out there and show off that big ol' brain like a clever version of a "Baywatch" lifeguard. It's one asset that won't last forever.