How do we really know that we (and everything else) wasn't created an instant ago, and everything we remember from the past is an illusion, a figment of our imagination to comfort ourselves from seeing and remembering the truth that all is not as it seems?
It depends what you mean by "instant" and "past". Both these terms derive from measurements that are taken from the reality we experience. There is no such thing as an "age" that something might "really" be other than we experience it because the entire concept of age is derived from measurements of the universe as we experience it. If we are to doubt the truth of that experience then we must apply the same doubt to the concept of time itself. We have no justification for believing one but doubting the other.