It seems pretty clear that according to the literal sense of Gen. 2-3, Adam and Eve were created as adults. If anyone had come along at the moment after their creation, and tried to discern how old the world was from their age, he'd have gone 30 years too far back, maybe. Preserving the fact that evolutionary theory, modern geology, etc., are probably correct, is there any reason we couldn't speculate, at least as a possibility, that the earth was also created as an "adult", analogously to the formation of the first man and woman?
God can pretty much do what He wants.