Still others believe the Moon is a big rock that had been drifting around the universe for a long time before it got captured by Earth's gravitational field. If that is true, the Moon could be much older than the Earth. All we know for sure is that scientists don't know for sure. Therefore, the age of the Moon has absolutely no bearing on the age of the Earth. But let's assume, just for the sake of discussion, that the age of the Earth is the same as the age of the Moon.
Scientists can smash a piece of moon rock down to individual atoms.
A mass spectrometer separates atoms by weight and tells you how much of each isotope is in the rock. It just tells how much of each isotope is in the rock nothing more. Certain radioactive isotopes turn into more stable isotopes at a known rate. For example, uranium turns into lead very slowly. Scientists can measure the amount of uranium and lead in a moon rock and figure out how much uranium and lead will be in the rock a billion years from now.