We don't really have a way of knowing exactly how old the planets are unless we can get hold of a sample rock from the planet and use radioisotope dating methods. We have dated the age of Earth, Moon, and other meteorites; the oldest ages are about 4.5 billion years or so. Astronomers assume that all the planets are about the same age within a few million years. We can use the Moon's dated history as a guide and roughly sketch out the history of the planets and their moons based on cratering frequency. We can say that the planets' ages must be consistent with the grand cratering era which occurred around 3.5 billion and 4billion years ago. When the planets' moons all show the same effects of the heavy bombardment era to about the same degree, so they are all at least OLDER than this cratering era; however, they should not be older than the Sun or the oldest meteorites we know of.