Those who were never exposed to the message still know the difference between right or wrong, read Romans chapter 2, their conscience bears. With infants, there comes an age of accountability, what is that age, I don't know, but Jewish tradition, if I remember correctly, 13 is when the son becomes a man. The mentally infirm, I guess you have me on that one, or it's not one I'm going to debate. We are accountable for our own salvation, we have more people in this world who look for excuses not to find God because it's the easier path, rather than to find God. If one will just look for God, He is not hard to find. When it all boils down, we all believe in something we cannot prove. He is just one step away.
Romans 3:11 says that, "Not one is seeking out God." Is this anyone's fault? No. Romans 11:32 says, "God locks up all together in stubbornness." We are all born stubborn, and apart from divine intervention, we stay that way.