How do scientists know how old it is?

Some rocks hold tiny amounts of radioactive stuff, which slowly turns into something else. Scientists know how long that change takes. So by measuring how much has already changed, they can work out the rock's age. It's like a clock that started ticking when the rock was made.

Why not just find Earth's very first rocks?

Earth's ground is broken into giant plates that slowly move, and over billions of years they've recycled the oldest rocks. The oldest bits left are tiny crystals from Western Australia, about 4.3 billion years old. Space rocks called meteorites formed along with the planets and stayed unchanged. They're about 4.5 billion years old.

How old is the Earth?

Zib answers for ages 8–11, then goes 2 steps deeper each time a child asks why.

Zib says

Long ago, the young sun had a huge cloud of gas and dust swirling around it. Gravity pulled bits of dust and rock together, and they clumped into bigger and bigger lumps. One of those lumps grew into our Earth. That was about four and a half billion years ago.

For ages 8–10

Checked against Earth facts (NASA Science)
How do scientists know how old it is?

Some rocks hold tiny amounts of radioactive stuff, which slowly turns into something else. Scientists know how long that change takes. So by measuring how much has already changed, they can work out the rock's age. It's like a clock that started ticking when the rock was made.

For ages 9–11

Checked against Age of the Earth (USGS)
Why not just find Earth's very first rocks?

Earth's ground is broken into giant plates that slowly move, and over billions of years they've recycled the oldest rocks. The oldest bits left are tiny crystals from Western Australia, about 4.3 billion years old. Space rocks called meteorites formed along with the planets and stayed unchanged. They're about 4.5 billion years old.

For ages 10–11

Checked against Age of the Earth (USGS)