We know that without the law there is no sin. That said, a person is capable of understanding the law at a certain reference point thus making him guilty of his own sin, sin being a willfull disregard for what one knows to be the law of god. If one offends the law in one point he is guilty of all- and faces a literal hell. This refrence point is termed age of accountabillity and is different for each individual. When one is capable of sin he is capable of chosing not to sin, thus mentally able to consent to actions. Emperically, We know that children are sexual beings since they are capable of arrousal, and so long as the information is laid upon the table in a clear format, how is one responsible for another's use of information or lack thereof? If that responsibility be the case, then one could never be of account thereof.