Why eight year old minimum for group lessons?

     I require art student to be at least eight years old to take group (4-8 students) lessons. Students between the age of five and eight can participate in private instruction.  I feel that those under the age of eight require more attention and are not as independent. Their shorter attention span requires instructions to be broken up into smaller segments and that takes more time. Younger students get tired much faster; have a harder time keeping silent and staying in their seats. Children under eight have less physical dexterity and mental development than what I expect for group lessons. Young children often do not have the emotional growth or understanding with the critique process used in group classes. I can accommodate for all these different issues in a private class.