I don't have a problem with the "G-d created a mature universe" theory, but it doesn't make sense to me that the mature universe would have signs of ancient civilizations far older than the 5766 years allowable by biblical chronology. And if all of civilization was wiped out by the flood, how did the cave paintings get there? (People who believe in a 5766 year old universe - can you explain?)
I can't answer that, I'm not in the ~6000 year old universe camp, and someone seems to have turned me into a newt.