[steven]

Wow, thanks for that breakdown. What you're saying makes a lot of sense to me.

However, I have a question about this:
"And there's pretty strong evidence for a point of no return as well; children who are never exposed to language at all and reach pre-teen age pretty much never learn to speak with normal clarity or use grammar correctly..."

By this do you mean that it's a point of no return for an L2 or a point of no return for language aquisition alltogether (as in an L1 is never completely formed)?

Edit:
In my experience, course outlines state way more than what actually gets accomplished. In other words, the course may be designed to have all the students pass 2級 for the Japanese efficiency test, but it seemed like everyone was more suited for level 3. And they say you'll learn 500 words or 2000 words or whatever... when in actuality you are exposed to them. So unless you have a 100% rate of remembering what's taught to you, you will not know that many words. Besides, even if you "know" a word, you might not be familiar with most of its usages- and while most of those alternate usages might seem obvious to a native they can be quite challenging to a learner. I agree with Columbine's notion that most courses are very "artificial". That's a very good way of putting it. No matter how good something looks on paper it's not a guarantee that it will work in the real world. Japanese courses seem to be a great example of that in my opinion.