Babies Understand Grammar

So why don't high school students?  According to IO9.com,  Jill Lany asserts that babies understand grammar before they learn how to talk.  She doesn't answer the question about high school students, however.  She does make me feel like a rotten teacher when she asserts,

Babies are constantly looking for language clues in context and sound. My research suggests that there are some surprising clues in the sound stream that may help babies learn the meanings of words. They can distinguish different kinds of words like nouns and verbs by information in that sound stream.

If babies can distinguish between nouns and verbs, high school freshmen should be able to as well.

Lany speaks about her research in this YouTube video: