I would disagree. I sin every day. I don't always realize that what I'm doing is sinning, but it is sin nonetheless and it separates me from God. Repentence from that sin, which implies a recognition of sin, is what God wants.
Remember, the Fall was related to eating from the Tree of the Knowledge of Good and Evil. Until Adam and Eve were able to "know" what was right and what was wrong, there was no sin.
I would say that the concept of "evil" had never entered their mind before then. They weren't aware of sin because they had never sinned up to that point. When they sinned, it became clear in their mind just what it was.
You may not always realize sin as you are doing it, but you have the capability of realizing it. Many factors prevent you from avoiding sin, but your conscience is what knows right from wrong.
I have no doubt that you are capable of telling right from wrong, therefore you are capable of sin.
Now, the mentally challenged or the very young do not have this capacity.
SD