4.6 billion years
The oldest rocks that scientists have found are 4.3 billion years old from northern Canada. However they have found meteorites that are 4.6 billion years old. These dates are determined by radiometric dating. There is consensus among scientists that radiometric dating is reliable despite what religious fundamentalists claim.
Even without radiometric dating we know the earth is a hell of a lot older than 6000 years. For example, scientists have found coral that is 380 million years old. They determined the age by counting 400 hundred daily growth rings in one annual ring. In other words, when the coral was alive and growing there was 400 days in a year. The earth's rotation is slowing down due to tidal friction. Scientists calculate that 380 million years ago there were 400 days in a year. The radiometric date of the coral gave the same age.
Anyone who believes the earth is 6000 years old shouldn't even bother with science. That premise would invalidate much of what science has discovered. It would mean human efforts to understand this world are futile. Meaning, God would have planted so many false clues that true understanding would be impossible. We would simply be at his mercy to believe whatever he tells us.