How old is the Earth? Scientists believe that the Earth is about four and a half billion years old. That’s pretty old — but not as old as the rest of the universe.
Back in 1862, Lord Kelvin calculated how long Earth might have taken to cool from its original molten state. He concluded that Earth was born 20 to 400 million years ago. Then scientists learned about radioactivity — the decay of atoms of one chemical element into another. That let them date the ages of rocks — and suddenly Earth got much older.
Therefore, How old is the Earth?
The oldest rocks on Earth are now thought to be around four billion years old. That would be the age of Earth’s solid crust. The planet itself probably formed about four-and-a-half billion years ago. As for the rest of the universe — most astronomers nowadays favor the Big Bang theory. That’s the idea that not only matter — but also space and time– were born in a primordial explosion.
The Big Bang theory isn’t proven, but astronomers say that it’s likely — so they think that the universe is about 14 billion years old. So — to answer the question how old is the Earth in contrast to the rest of the universe — that’s about 14 billion years for the age of the universe — and about 4 1/2 billion years for the age of planet Earth.
Nobel e-Museum (A wonderful resource) – The Age of the Sun