Carbon-14, the radioactive isotope of carbon used in carbon dating has a half-life of 5730 years, so it decays too fast. This makes it ideal for dating much older rocks and fossils.

Why is carbon used for dating fossils of once living things?

Radiocarbon decays slowly in a living organism, and the amount lost is continually replenished as long as the organism takes in air or food. Because carbon-14 decays at this constant rate, an estimate of the date at which an organism died can be made by measuring the amount of its residual radiocarbon.