I recall hearing this before, and I believe that the reasoning was that children are not developmentally ready for the abstraction of algebra. If I recall correctly this is why my gifted classes didn't present pre-algebra before age 11 (when I was in 6th grade).
However, I know that DD's elementary curriculum specifically exposes kids to some algebra concepts as early as 1st grade.
I'm not sure if the original idea was based on Piaget's developmental stages or not.
I recall that it is a developmentally ready issue too. I think someone who teaches algebra can test to see if a child is developmentally ready instead of just assuming readiness or not based on age. I think the readiness age could just be used as a typical guideline.
I would look into what criteria concludes they are ready. Let us know if you figure it out.
