Our home planet is about 4.5 billion (4,500 million) years old. This date is based on samples from the Earth, meteorites and the moon, which all come from when the Earth first formed. Scientists have looked at radioactive chemicals in the rocks which react into new chemicals over a certain period of time. By working out how many times this reaction has taken place, scientists are then able to determine the age of the rock.
Answered by David Gelsthorpe, Manchester Museum.