The age of the earth is difficult to determine because our best evidence comes from radioactive decay rates observed in solid rock, and the effects of weathering and plate tectonics have obliterated nearly all surface rocks less than three billion years old. The oldest rocks discovered so far are between 3.5 and 4 billion years old, but all of these rocks are from lava flows and shallow-water sediments. These lavas and sediments must have flowed and fallen over still older rocks which have since been geologically recycled. Single zircon crystals, eroded from their original rock formations and deposited as more recent sedimentary rock, are the oldest terrestrial minerals yet found; they are up to 4.4 billion years old and currently set a minimum age for the solid crust of the earth.

According to current theory, the moon was formed in a collision between the proto-earth and a Mars-sized object. Most of the impacting mass became part of the earth, which was significantly larger as a result, but parts of both objects were splashed into space, some of which coalesced and formed the moon. The oldest moon rocks recovered so far are 4.4 to 4.5 billion years old, setting a minimum age for the formation of both the moon and the full-sized earth. Any pre-existing solid crust on the proto-earth would have been melted by the impact.

Some ancient meteorites have been dated at between 4.53 and 4.58 billion years old, representing the date at which the uranium and lead used for radiometric dating were first incorporated into solid rock (the planetesimals when then accreted to form the planets).

The accretion time from planetesimals to planets was a few tens of millions of years, giving an age for the proto-earth (probably completely melted) of just about 4.5 billion years. The impact that created the moon occurred shortly thereafter and would then have been the last major accretion event for the earth. The oldest possible rocks on earth formed the original crust after the impact melted (or remelted) the planet.
Source(s):
http://pubs.usgs.gov/gip/geotime/age.html
http://en.wikipedia.org/wiki/Giant_impact_hypothesis
http://en.wikipedia.org/wiki/Planetesimal
http://www.deepfly.org/TheNeighborhood/7c3-Evolution.html

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