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Thank you for your interesting question.
I found a summary of a study done in 2005 where the age of all the various tissues was determined by looking at the presence of the radioactive carbon 14 isotope (14c) in the DNA.
The ages of the different tissue does vary quite a bit, but when averaged comes to approximately 7 years, as has been taught for years.
The age of skeletal muscle tissue was found to have an average age of 15.1 years.
In the summary I read the age of skin was not mentioned, but the epithelial cells of the intestine have a very short life span of only 5 days, so my guess is the the epithelial cells of the skin is fairly short as well.
Other interesting facts are the the average age of the cerebellum's gray matter was found to be only 2.9 years, the average age of the occipital-cortex gray matter was about 10 years younger than the individual, but the age of occipital-cortex neurons was the same age as the person.
Spalding KL, Bhardwaj RD, Buchholz BA, Druid H, Frisen J. (2005) Retrospective birth dating of cells in humans. Cell. 122:133-43.
I hope this is satisfactory!
If not, let me know and I'll do more research.