The Earth is old. Real old. Sometimes it is hard to truly grasp just how old it is compared to human lifespans, but if we say time represented in a century is the same as the length of a football field (100 yards or 91 meters), then all of Earth's history would get us to the Moon and back over five times! So, when Earth scientists talk about the oldest stuff on the planet or in the solar system, they're talking about rocks and minerals that reach that far back (or more) into Earth's past.
When we talk about the age of the Earth, we're really thinking about the age of the solar system as well. The Earth formed from the spinning cloud of dust and gas that formed our Sun and all the solid debris around it. If we want to know how old the planet is, we need to think about what we would date (usually using radioactive elements and their decay) to get us back that far.
