Just a quick chime-in because I agree that you don't have to wait for a PG child who understands abstract thinking....
DS6 is only at GL 3.5 on EPGY, but I love that they have him practicing algebraic equations and teaching how to substitute equations and solve for the variables. When he started a few weeks ago, he was at 3.0, and it took him all of 5 minutes to figure out that using letters to solve was the same thing as spaces, question marks, part-whole equations, and everything you'd see in other math programs. Now that he's having to solve with multi-step equations, he thinks it's just a fun puzzle. He is having so much fun with the problems that "algebra" is now his favorite part of math!
For elem. children who like mind benders and that sort of thing, algebra seems like a natural extension.
