Xxxonline inx - Radiometric dating of planets
The United States Geological Survey(USGS) website has a lot of indepth material about how the age of the Solar System was determined.
The basics of it are that all material radioactively decays into a stable isotope.
The begats in the Bible were another source for estimates: In the 17th century, Ireland's Archbishop James Ussher reconstructed the genealogy of biblical figures and declared that Earth was created at 6 p.m. The modern effort to understand the age of the planet started with Nicholas Steno, a Danish anatomist and geologist who was among the first to realize that fossils are the remains of living creatures.
He proposed that geologists might learn about Earth's history by sifting through layers of rock, which were laid down over the course of millennia and provide a backward chronology of our planet.
The work, led by geochemist Ken Farley of the California Institute of Technology (Caltech), could not only help in understanding the geologic history of Mars but also aid in the search for evidence of ancient life on the planet.