"The mother tongue alone is well pronounced because it was established in the period of childhood; and the adult who learns to speak a new language must bring to it the imperfections characteristic of the foreigner's speech: only children who under the age of seven years learn several languages at the same time can receive and reproduce all characteristic mannerisms of accent and pronunciation."  Maria Montessori, The Montessori Method, Chapter 18, p.315-16, Schocken Books, Inc., 1964.