You know I have always believed that those that do not understand they have sinned against G-d are still "innocent" without sin before G-d so if they die they will go to heaven, the children and mentally ill. According to Ezekiel chapter 16. Tradition is the age of Bar/Bat Mitzvah. But I real an article once on how G-d did not punish the children of Israel upto the age of 20, anyone under 20 could enter the promised land. Do you think there is an age of "accountability"? what age? or what qualifies?