The language instinct.
One of the things about studying how language works for a living is that you quickly discover that everyone has an opinion. Not only that, it's like religion rather than like astrophysics: everyone has an opinion that they consider to be valid. Because everyone speaks some language, everyone thinks that they know how Language works.
In one sense, they're right. In this sense, people know as much as adults as they did as children. People have brains particularly well-equipped for learning language; something about our cognitive architecture is perfectly suited to the learnability problem posed by human languages. The relevant cognitive architecture is not shared by other species, even those most closely related to us. We're significantly better at applying this kind of knowledge when we're children, for reasons that no one completely understands but must have something to do with the way the brain changes as we age. This is the knowledge that allows people to say with relative ease whether a particular sentence is grammatical or not in their language or dialect. (Well, unless that sentence is the convoluted construction of a syntax paper. But that's another topic for another day.)
But this isn't the kind of knowledge people think they're calling upon when they think their kids are being polluted by the poor grammar of the other kids in their kindergarten classroom; when they express vociferous opinions about Ebonics and the Oakland school board resolution, which they may or may not have read; when they talk about Latin being the most logical language of all; or when they argue that Spanish-English bilingual teachers won't be able to teach science effectively. There are more myths about language than there are about UFOs. And they're more pernicious, because fewer people recognize them as myths.