To figure out the age of the earth, it is important to know how old the rocks on it are. Scientists can date rocks by measuring the amount it of radioactivity, or rays of energy, they give off.
All rocks contain some uranium, which causes radioactivity, and that uranium give off invisible rays of energy which slowly change the uranium into lead. Scientists know how long it takes for uranium to change into lead, so by comparing how much uranium and how much lead are in the rocks, they can determine the age of the rocks.
By this method, scientists have determined that some granite gneiss rocks found in Tanzania, East Africa, are about 3.5 billion years old!
But bits of meteors that have fallen to earth are even older. They are parts of other planets in our solar system, and seem to be 4.5 billion years old! Most scientists believe that our whole solar system started off at the same time. We can therefore figure the planet earth to be about 4.5 billion years old.