Infants exposed to two languages (e.g., Japanese and English) in the first seven to eight months of life will easily develop the neuron functions that can differentiate the sounds of the two languages. This sets a base for fluent mastery of both languages without an accent later in development. Individuals who develop capability for two languages early in life have a larger left temporal hemisphere of the brain than do individuals with monolingual backgrounds. This may be, in part, an explanation of why those individuals can also more easily master other languages later in life. (Mustard, J. Fraser, MD. Early Childhood Development: How does experience in early life affect brain development? p 12. 2008.)

Children who grow up alone or in the wild without exposure to language until age ten rarely if ever learn to speak. (Karr-Morse, Robin, and Meredith S. Wiley. Ghosts from the Nursery. p 22-23. NY: Atlantic Monthly Press, 1997.)