According to Maria Montessori's theory of education, the child moves through three planes of development in their lifetime: the first is from the ages of 0 to 6, the second from 6 to 12 and the last from 12 to 18. For classroom purposes, the planes are broken down further into the Children's House (ages 3-6), the Lower Elementary (ages 6-9), the Upper Elementary (ages 9-12) and the Middle School (ages 12-14).