Radiocarbon decays slowly in a living organism, and the amount lost is continually replenished as long as the organism takes in air or food. Once the organism dies, however, it ceases to absorb carbon-14, so that the amount of the radiocarbon in its tissues steadily decreases.

How is c14 used to date cells?

Radioactive fallout from nuclear tests serves as measuring stick. But when it dies, this ratio will fall, as the carbon-14 decays. Radioactive carbon decays slowly, such that a given amount of carbon-14 halves every 6,000 years.