Cell life span in the human body: 7 years?

Karel & Iris Schrijver “Living with the starts” have an intestine chapter “Dying to Live” that is about the cell turn over

Take the skin, for example: a Iiving, breathing, regenerating tissue that is the largest organ of the body and that acts as a barrier between the internal organs and the environment. In adults, it encompasses about 22 square feet (2 m2) and weighs around eight pounds (4 kg). It protects the interior of the body from injury, from harmful effects of microorganisms, and from the damaging ultraviolet rays of the Sun. It plays a role in the body’s thermal regulation through the constriction or dilatation of small blood vessels, it contains nerve endings that allow us to feel touch, temperature, pain, pressure, and vibration, and it slows the loss of fluids from the body. The skin also shelters the hair follicles, which produce the hairs that cover most of the body’s surface, and it provides storage for a variety of substances. The skin, composed of several layers, ages quickly but is remarkably effective at renewing itself. In the top layer, the epidermis, most cells eventually reach the surface as the outermost layers at cells wear off. They are replaced in a time frame of roughly a month or two, in a continuous process that culminates in the loss of approximately 30,000 cells every minute throughout our lives. This translates into roughly eight pounds (4 kg) of dead material per year. Some features of our skin are, of course, more lasting. For example, we may have seemingly permanent moles and we may have scars that persist for years. These tissues, however, are not really skin. Moles are embedded within our skin but they are in fact benign growths that are typically composed of pigmented cells that do not follow the same lifecycle as true skin cells. Likewise, scars are repairs of deep cuts in our skin.

Unfortunately they do not give any reference there. Data should have been produced by carbon 14 dating. And yes, there are references

Radioactive carbon decays slowly, such that a given amount of carbon-14 halves every 6,000 years. So detecting the subtle change in the ratio of normal to naturally occurring radioactive carbon over just a few years is incredibly hard.
But Jonas Frisén of the Karolinska Institute in Stockholm, Sweden, says it can be done if one takes advantage of the signal left by nuclear testing, which spewed high levels of carbon-14 into the air during the Cold War.
By the time a halt was called to aboveground nuclear testing in 1963, levels of carbon-14 in the atmosphere had doubled beyond natural background levels, says Frisén. Since the halt, this has halved every 11 years. By taking this into account, one can see detectable changes in levels of carbon-14 in modern DNA, he says.
“Most molecules of the cell will turn over all the time. But DNA is a material that does not exchange carbon after cell division, so it serves as a time capsule for carbon,” Frisén says.

basically referring to a Cell 2005 paper

We therefore modified established DNA-extraction protocols to minimize the risk of carbon contamination (see Experimental Procedures). DNA samples were analyzed for purity in several ways; in addition to spectrophotometric analysis, the contents of all samples were analyzed by HPLC and the amount of total carbon (12C, 13C, and 14C) was determined during graphite preparation for isotope analysis by accelerator mass spectrometry.

They needed a minimum of 15 million cells for 14C analysis with the current sensitivity of accelerator mass spectrometry while it would be  interesting to repeat this study with single cell genome sequencing.