I really don't see a reason why there should be an age limit for starting algebra. DS8 could solve simple algebraic equations at the age 5 without ever being taught the material. A year later he could understand logarithms.
We moved from Singapore directly to AoPS. I originally planned on doing NEM first, but AoPS Intro to Number Theory seemed like such a perfect match for DS8 that we started that instead. It's exactly the type of math he loves. At one of the DYS parent seminars I asked Richard Rusczyk about doing Intro to Number Theory immediately after Singapore and before Algebra and he had no objections.
So far we've been very happy with the AoPS material. DS8 doesn't take the class, but we cover the material as part of our homeschooling curriculum. We will probably start Intro to Algebra next.
