1. How do you determine the relative age of a planet such as Jupiter?
Models for the formation of the planets show that all of the planets formed within the first 100 million years of the solar system, maybe less. Jupiter does not have a solid surface, so there is no other way to determine its relative age using features such as impact craters. For the terrestrial (i.e. rocky) planets, one does crater counting to get the relative ages of the surfaces (not the age of the object). Mars may get hit a little more often because it is closer to the asteroid belt whereas outer Solar System satellites may be getting hit more often because there are more comets. But, when one compares different parts of a surface or one Jovian moon to another, one can get relative surface ages from crater counting.