Today on Do Some Damage, he resurrected the discussion, adding the side question, "[A]t what age do you stop teaching [grammar] and expect kids to know it?"
To me, grammar is vitally important to clear thought. It's easy to jump on the bandwagon that the meaning behind a sentence is more important than how the sentence is structured, but grammar helps to teach meaning in the first place. If a person doesn't get that, he will have a more difficult time communicating his ideas—his meaning—later.
It's also easy to say that grammar isn't as important in fiction, but I think the grammar for fiction is simply different depending on the world you're writing about; it's no less important for your writing to be well structured.
I think everyone should know the parts of speech by seventh grade, but as a teacher I didn't see that. So I don't think we can have an age by which grammar is no longer taught, but then learning grammar is learning to communicate, something we continue all our lives.