C-14 is produced when high-energy particles from solar radiation hit the earth’s atmosphere and make the unstable element called C-14.
As time goes by, this C-14 slowly changes back to stable atoms.
That causes a dating problem with any animal that eats seafood. After about ten half-lives, there's very little C14 left.
So, anything more than about 50,000 years old probably can't be dated at all.
Pope Francis sent a special video message to the televised event in the Cathedral of Saint John the Baptist in Turin, Italy, which coincided with Holy Saturday, when Catholics mark the period between Christ's crucifixion on Good Friday and his resurrection on Easter Sunday.
It is produced in the upper atmosphere by radiation from the sun.
(Specifically, neutrons hit nitrogen-14 atoms and transmute them to carbon.) Land plants, such as trees, get their carbon from carbon dioxide in the air. The same is true of any creature that gets its carbon by eating such plants. Suppose such a creature dies, and the body is preserved.
The C14 will undergo radioactive decay, and after 5730 years, half of it will be gone. So, if we find such a body, the amount of C14 in it will tell us how long ago it was alive. The method doesn't work on things which didn't get their carbon from the air.
This leaves out aquatic creatures, since their carbon might (for example) come from dissolved carbonate rock.