Carbon-14 is a radioactive isotope of carbon that is naturally incorporated into living organisms. After death, Carbon-14 decays at a predictable rate with a half-life of approximately 5730 years, allowing measurement of the time elapsed since death. This makes it ideal for dating ancient biological materials such as bones, wood, and textiles. Carbon-12 and Carbon-13 are stable isotopes and do not decay, thus cannot be used for radiocarbon dating. Carbon-11 has a very short half-life (20 minutes) making it unsuitable for long-term dating purposes. Therefore, Carbon-14 is uniquely useful for determining the age of archaeological and geological samples up to about 50,000 years old.

Reference: Robbins & Cotran Pathologic Basis of Disease, Kumar, Abbas, Aster, 10th Edition.