The heart is the spiritual center of man's being. The sinful heart is only evil, and anyone who has one of these hearts is evil. All day long, every day, the whole of man's heart is evil. It is constantly evil. The Bible does not teach the Age of Accountability.
Children are sinners as much as anyone.
They are sinners from conception and do not receive special treatment from God just because they are babies or children. The depravity, or sinfulness of man is a doctrine which is vital to understanding salvation. Because most of the church today does not understand the doctrine of the sinfulness of man. People think that man still has the capacity to be good. They think people are able, in and of themselves, to do works of righteousness before God without the help of God at all.