So God made man on the first day. He didn't make them babies, but made them adults, just like he made them old he made the earth old.
Actually God made man on the 6th day. But that's not what we are debating. But Adam and Eve had to be able to take care of themselves so they couldn't have been babies, but I wouldn't say they were old...probably just barely adult age.
I think your "God made the earth old" theory goes along kind of with what I said earlier about God taking matter from other places to create the earth with. Because the matter he took from other places had to be old it couldn't be new, and that's why it possibly had dinosaur bones and stuff in it.
I just wanted to add one more thing I've been thinking about. And that's that we don't know how long Adam and Eve were in the garden of Eden before they both ate the forbidden fruit and got kicked out. They could have been in there for like 5 billion years...who knows? Obviously you have to believe in Adam and Eve for this to even apply.
Oh and one more thing, pertaining to the comment about Cavemen. I keep seeing these shows on the History channel or somewhere about how Cavemen and regular men existed together as two different species that had nothing to do with one another. I don't really follow it. But is it possible that somewhere along the way God cursed a group of regular men and made them into cavemen for disobeying?