How old?
One of the chief ways scientists determine an age for the earth is by measuring the rates of decay of radioactive isotopes.
Within the Earth's crystalline rocks, daughter isotopes are found alongside their radioactive parent isotopes (including whole decay chains), scars (radiohalos) are visible due to the a-particles produced by radioactive decay, the a-particles themselves have been retained as the nuclei of helium atoms, tracks are visible from decay by fission, and the heat produced by radioactive decay is present.