There are several reasons as to why the scientists believe that the Earth is older than the age of the oldest rocks. One of those clues is that the oldest rocks do not correspond with the estimated age of the formation of the planets in the Solar System, thus the Earth as well. Another one is that the Earth was initially hot, gassy and liquid, and it took millions of years for it to cool off, so only after its surface cooled off enough, the Earth got its first rocks. The rocks are also not staying static, but instead they move because of the tectonic movements, so over the course of millions of years, they weather, erode, sink, subduct, all of which can wipe them off the face of the Earth, so the younger ones remain.