Each is important, but I believe the fifth year is often the most critical. Up to that time, a child believes in God because his or her parents say it is the right thing to do. She accepts the reality of Christ like she would a story about Santa Claus or the Easter Bunny--uncritically and innocently. At about five or six years of age, however, she begins to think more about what she is told. Some kids come to a fork in the road about that time. Either they begin to internalize what they've been taught and make it their own or else the Bible stories become like the fables that don't exist in the real world. It is a time for careful instruction at home and in church.
I certainly don't mean to imply that parents should wait until the child is five or six to begin spiritual training. Nor are subsequent years insignificant. But I am convinced that our most diligent efforts within the family and our best teachers in Sunday school ought to be assigned to the child of five or six years. There will be crucial crossroads after that, but this one is vital.