Geologists now think the Earth is about four and a half billion years old.
This is known from radiometric dating of rocks. In radiometric dating, scientists measure the amounts of different isotopes of certain elements in a rock sample. Since radioactive isotopes are known to decay at a certain constant rate, the amounts remaining in the rock will tell how old it is. For igneous rocks this is the time when it solidified from molten magma. The oldest rocks ever found on Earth are actually slightly younger than this, about 3.8 billion years old.