One of my kids did high school geometry at eight (per his request) and then algebra at nine. He did exceptionally well on the SAT math portion at 12, and started college math/science courses at 12. He might have been more radically accelerated, but we chose not to have him rush through the typical math sequence. He spent a lot of time doing things like watching Teaching Company lectures and playing around casually with Life of Fred rather than working in a more linear fashion.