Moorad,
Yes, you are right about this being a fundamental and important assumption.
I think much of the debate over ID surrounds the question of testability
of "miracles" or how we identify God's hand in creation. We must all how a
lower threshold of how much appearance of age we can accept. For example,
just taking the basic assumption of the example given to its the extreme we
could ask "how could you determine that the world existed before, say, the
Flood at all?" Couldn't it be that God created all things at that point
with Noah just getting off the boat and the memories of the times before
that in his head? Isn't that similar to Adam created with language and
culture? I would say no but I certainly can't prove that scientifically.
What I can do is shed doubt on that hypothesis with theological arguments
concerning what we know about God and how we can expect him to act. From
his Word I think we can understand that this would not be in the character
of God to have created a world that began with Noah (or even yesterday for
that matter).
regards,
rjd