This is a term that is not in the Bible, but some religious groups use to refer to the time when a child becomes accountable to God for his or her actions. That time comes whenever he or she is old enough to understand the difference between right and wrong, good and evil.
It may be at a very early age. Some children who are four or five years old are old enough to know Jesus Christ and be saved, because they are old enough to know the difference between right and wrong. We have to be careful with little children. We should not cause them to build up an unnatural sense of guilt and make them feel guilty for things that are not really sin at all. If we teach them the major moral rules, that God Himself has set forth, then they will learn properly and naturally.
Children do reach an age where they are old enough to be held responsible for what they do. When that time comes, they will be judged by the same standards as everyone else.