Carbon-14 is a radioactive isotope and is present in all living things in a constant amount.
Because of the carbon cycle, there is always carbon-14 present in both the air and in living organisms.
Carbon dating was used routinely from the 1950s onward, and it confirmed the age of these historical remains.
Radiocarbon dating is a method used to date materials that once exchanged carbon dioxide with the atmosphere; in other words, things that were living.
Scientists often use the value of 10 half-lives to indicate when a radioactive isotope will be gone, or rather when a very negligible amount is still left.