Now the Bible does refer to an age of accountability. As a smaller child, most can't grip this stuff. Depending on the degree of someone being handicapped, they cannot grasp this either.
Sin is in man as a whole. Its not a one time thing and you are going to hell. Its whats in us. Man can do some great things, but its in our hearts to sin. Watch a small child. They know how to lie without being taught. They can get really angry and slap and bite. They get jealous. You can see it all from an early age.
The examples that you provide are excellent. However, you gotta look at the over all picture. We know when we are doing something wrong usually. There are some areas that we don't realize that are wrong, but we all have the basics down. In an average life time, we'll face many, many things that we know what we are doing.
IMO, many people think that hell is unfair, but if you come at it from a different perspective, you can see why its there. People think that God just wants to send people to hell for being bad. However, he wants everyone to come to repentance and not to go to hell. = And I think this is the area where the hell topic gets exciting.