Based on our study of meteorites and rocks from the Moon, as well as modeling the formation of planets, it is believed (pretty much well-established) that all of the objects in the Solar System formed very quickly about 4.56 billion years ago.When we age date a planet, we are actually just dating the age of the surface, not the whole planet.For example, a geologist may examine a cutting where the rocks appear as shown in Figure 1.Here he can see that some curved sedimentary rocks have been cut vertically by a sheet of volcanic rock called a dyke.It is clear that the sedimentary rock was deposited and folded before the dyke was squeezed into place.
The different methods of radiometric dating are accurate over different timescales, and they are useful for different materials.
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.
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.