We have rocks from the Moon (brought back), meteorites, and rocks that we know came from Mars.
We can then use radioactive age dating in order to date the ages of the surfaces (when the rocks first formed, i.e. We also have meteorites from asteroids and can date them, too.
We have an activity in one of the PSI workshops "Exploring the Terrestrial Planets," that deals with this topic.
Note that it would be extremely unlikely for another dating method to agree with these bogus ages.
Getting agreement between more than one dating method is a recommended practice.
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.
The number of protons usually determines the element the atom belongs to and it is fixed for any particular element.
On the other hand, the number of neutrons that can be contained in the nucleus can vary.
When the number of neutrons is in balance with the number of protons (which does not necessarily means that the number of neutrons has to be exactly the same as the number of protons) then the atoms of a particular element is said to be stable.
Example: The radioactive element carbon-14 has a half-life of 5750 years.
The percentage of carbon-14 present in the remains of plants and animals can be used to determine age.
The biggest assumption is that, to first order, the number of asteroids and comets hitting the Earth and the Moon was the same as for Mercury, Venus, and Mars. The bottom line is that the more craters one sees, the older the surface is.
This can be interpreted in two ways: why it is important to know the age of a planet or how is age dating important in determining the age of a planet?
These are the surfaces that we can get absolute ages for.