I'm going to quote from Alfred North Whitehead himself in answering this -- from his "Rhythm of Education" speech (from 1922). He said: "Different subjects and modes of study
should be undertaken by pupils at fitting times
when they have reached the proper stage of mental
development." Then he added, "I
do not think that this obvious truth has been
handled in educational practice with due attention
to the psychology of the pupils."
In another essay of his, "The Aims of Education," Whitehead also listed these two "commandments" for education: "Do not teach too many subjects," and "What you teach, teach thoroughly."
Basically he advocated teaching students at an appropriate depth for their place in the educational process -- and he believed in pushing them to learn hard things soon in the process when those are necessary things in order to keep learning and growing.
