Spend Time Together
In the past, a teacher would not easily choose a student until they found someone who was good enough. If the student was a young child, child would leave his or her family and go to live with the teacher. The parents would allow this and be happy for their child to be chosen. Of course the children would return home on special occasions to see their parents. It was a little like a boarding school today where the children would learn skills like carpentry, ceramics, cooking and study for maybe 3 to 5 years. After they graduated they could then work for others or work for the teacher.
For martial arts, it will take much longer, at least 5 to 7 years or even longer. So the younger the students were the better. Of course, when staying with somebody a child will go through different relationship problems. Everyone is human and when they are in a bad mood or make some mistake a problem could happen. So the teacher would educate the student and make them understand and discipline them. The students could be away for at least 5 to 7 years and so their relationship with a teacher would grow very strong and they would bond and become like father and son.