http://www.sciencedaily.com/releases/2014/02/140213142235.htm
significant differences in both vocabulary and real-time language processing efficiency were already evident at age 18 months
By age 24 months, there was a six-month gap … in processing skills critical to language development.
Infants who heard more child-directed speech developed greater efficiency in language processing and learned new words more quickly. The results indicate that exposure to child-directed speech — as opposed to overheard speech — sharpens infants’ language processing skills, with cascading benefits for vocabulary learning.
