Carbon-14, or radiocarbon, is a naturally occurring radioactive isotope that forms when cosmic rays in the upper atmosphere strike nitrogen molecules, which then oxidize to become carbon dioxide.
Samples from the past 70,000 years made of wood, charcoal, peat, bone, antler or one of many other carbonates may be dated using this technique.
Older samples have a lower concentration of radiocarbon, but they can be (and often are) counted for longer periods of time to compensate for this lower concentration.
By counting longer, the counting uncertainty in a radiocarbon measurement on a very old sample can be the same as that on a young sample.
In Accelerator Mass Spectrometry (AMS), for example, the number of radiocarbon atoms in a stream of atoms coming from the sample is counted.
Thus there are statistical counting uncertainties proportional to the square root of the number of atoms counted.
A common misconception about radiocarbon dating is that it gives a precise date---3577 B. In actual practice radiocarbon dating can only give a range of dates for a given sample---3650 to 3410 B.