For others, all we are doing is getting a relative age, using things like the formation of craters and other features on a surface.
On the other hand, the number of neutrons that can be contained in the nucleus can vary.So, you can use the radioactive elements to measure the age of rocks and minerals. Their useful range is from about 1/10 their half-life (the time it takes for half of the radioactive element/isotope-- the parent, to convert into a non-radioactive element/isotope-- the daughter) to 10 times their half-life. You can use this to measure the age of a rock from about 128 million years to more than 10 billion years (the Solar System is 4.56 billion years old).So, Carbon-14 can only measure things up to just over 50,000 years old, great for determining when someone built a wood fire, but not good for determining the age of a meteorite. It occurs whenever an atom has an unbalanced number of protons and neutrons in its nucleus.This is also a way to get at the abundance of the various isotopes of carbon.We have an activity in one of the PSI workshops "Exploring the Terrestrial Planets," that deals with this topic.