How old is Earth? Earth is thought to be between 4.5 and 4.8 billion years old. The age of Earth is found by measuring the age of very old Earth rocks. This is done by measuring the rate at which elements of the radioactive metal uranium decay (break down) into lead. Scientists have also measured the age of meteorites which have fallen onto Earth's surface, and the age of moon rocks brought back by the Apollo astronauts. Both the meteorites and the moon formed around the same time as Earth, and they show similar ages.