Here's a video of Siegfried Engelmann, who is mentioned in this article, demonstrating how he had taught inner city grade school age kids that were considered "unteachable" (I can't recall the exact label I was told) doing algebra... live. (about 20 min in) I think they are about 4th or 5th grade.
https://www.youtube.com/watch?v=j9SjFsimywA
Many people didn't like the strict requirement of his reading methods, from what I have been told.