6-9 month olds know many words

http://www.sciencedaily.com/releases/2012/02/120213154057.htm

In research focused on 6-to-9-month-old babies, University of Pennsylvania psychologists Elika Bergelson and Daniel Swingley demonstrated that the infants learned the meanings of words for foods and body parts through their daily experience with language...


In fact, infants are often referred to as "pre-linguistic," according to Bergelson. But there have been few attempts to determine just when infants begin understanding what is meant by specific words. The belief that infants do not comprehend language for most of the first year is easy to understand, given that infants do not often speak in words, or even gesture meaningfully, before 10 or 11 months...

In both the two-picture and scene tests, the researchers found that the 6- to 9-month-old babies fixed their gaze more on the picture that was named than on the other image or images, indicating that they understood that the word was associated with the appropriate object.
This is the first demonstration that children of this age can understand such words.