Age dating relative
Different methods have their own limitations, especially with regard to the age range they can measure and the substances they can date.
A common problem with any dating method is that a sample may be contaminated with older or younger material and give a false age.
When we age date a planet, we are actually just dating the age of the surface, not the whole planet.
A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.
However, by itself a fossil has little meaning unless it is placed within some context.
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?