The amount of time it takes for an unstable isotope to decay is determined statistically by looking at how long it takes for a large number of the same radioactive isotopes to decay to half its original amount.This time is known as the half-life of the radioactive isotope.
In nature, all elements have atoms with varying numbers of neutrons in their nucleus.
It is impossible to predict when a given atom will decay, but given a large number of similar atoms, the decay rate on average is predictable.
This predictable decay is called the half-life of the parent atom, the time it takes for one half of all of the parent atoms to transform into the daughter.
These are the surfaces that we can get absolute ages for.
For the others, one can only use relative age dating (such as counting craters) in order to estimate the age of the surface and the history of the surface.
We can get absolute ages only if we have rocks from that surface.