The moons surface is covered in pieces of asteroids that have impacted the moon. This means that most of the moon rocks are just chunks of asteroids. These asteroids (called Chondrites) were around at the beginning of the solar system, so by measuring their age, we can get an estimate for the age of our solar system.
In fact, we can tell from these rocks that because of the concentration of certain radioactive isotoes in these chondrites, our solar system was triggered to form from a near by supernova event :)
