Explain the evidence for the age of Earth and why the oldest expected rocks will probably never be found.

The dating of zircon grains gives us the age of the earth as we know it.

Hi
To answer, the second part of your question, as to why the oldest rocks cannot be found. This is actually a tricky question to answer as a)you have to define the word 'rock' and b) What type of rock, you are trying to age.
If we look at the lithosphere, first and foremost, older rocks are destroyed as time progress's through plate tectonics. Where at a plate boundary, you have new plate being formed, on the other side you have plate being destroyed, through subduction. So for instance, the oldest sea floor rocks are about 250ma. As the older rocks have been destroyed.
Older rocks can be found to age the earth, through minerals that have been ejected from volcano's also, for instance lunar rock as this is geologically dead, rocks collected have been aged to 4.6 billion years, as there is no weathering of destructive plate boundaries. As we know, the moon is a product of a large planetary body, colliding with the earth in its early formation, ejecting the matter that is now locked to our orbit as our moon. The material ejected was at some point part of our world.
If we go further, we can tell the age of the solar system from rocks that are ejected from other planets like mars.. going deeper to the age of older bodies we can use asteroids and meteorites. Some rocks that land as asteroids are those ejected in earths violent past into space, that return and again can be aged.
Going back to our planet. As you may know when the earth was molten, the heavier elements, through gravity sank to the earths core and produced layers, as we know rocks are a collection of minerals, where as towards the centre of our planet we have elements (minerals) such as iron. The rocks, for which you speak of, in essence are those found in the lithosphere or the mantle. These can be aged quite well through modern methods.
All in all, the aging of our planet, is quite a complicated process, depending on what it is you want to age and where it has come from. I hope this assists you.
Kindest regards
Sean