There are many millions of craters on the moon and on Mars. Were all of these craters formed in the last 6000 years?
The moon was created with craters, just as the Earth was created with oceans and mountains and valleys. Another answer is that during the creation of the world, time passed very quickly, allowing for many changes that could have occurred over millions of years to happen within a few hours. This is a concept that is entirely possible according to Einstein's theory of relativity.
See this for information about the age of the world http://www.haemet.net/articles/creation/world_age/world_age_explanations.htm