"Looking down on children at the age of three" means that from the psychological characteristics and personality tendency of children at the age of three, we can see the embryonic form of children's psychological and personality image when they grow up. The age of three to seven is called infancy (entering kindergarten), also known as preschool.
The so-called "looking at the old at the age of seven" means that the child's personality tendency begins to form at the age of seven.