I'm no geologist but absolutely all of the material from which the Earth was formed existed before the planet was formed. In fact, all this matter was created shortly after the Big Bang (according to the standard Big Bang theory). The elements have slowly been formed in stars through nuclear fusion (and the heavier elements are the results of supernovas), which is why Carl Sagan could say "we are all made of star stuff".
However, when the Earth formed it most likely went through a period (or even multiple periods) where it consisted mostly of molten lava. It took quite some time for the planet to cool down and gain a planetary crust. Thus it will be difficult to find rocks that are as old as the Earth. We know, by observation and measurement, the approximate age of the Sun by measuring the ratio of hydrogen to helium. As the sun ages, it fuses hydrogen into helium. The age estimate for the sun is around 5 billion years. The sun had a protoplanetary disk (which can be observed around many other stars as well), out of which the planets eventually formed. Comets buzzing around our soular system have been there since the formation of the solar system, and are believed to contain the basic chemicals needed for life to form on Earth. this is the basis for the European Space Agency's Rosetta mission.
ESA: Rosetta Feature
Scientists have found fossils of living organisms as old as 3,5 billion years, and there is evidence for water almost 4,3 billion years ago.
Water on Earth 4.3 billion years ago
So, all in all, yes, scientists can very well estimate how old the planet is, and how it has evolved since the formation.
And PLEASE spell my name right.
Tormod
Your Friendly Neighborhood Administrator
