English Language Proficiency Test Question of the Day

"The period under five years of age is one of very rapid growth. There is probably no other time in the life history of the individual when the body and mind are so responsive to environment and impressions, and when so much can be done to build up good health as during the preschool age."

-Passage adapted from Healthy Children: A Volume Devoted to the Health of the Growing Child, by Sara Josephine Bake (1920)

What is the main idea of the above excerpt?