When I look at the test scores for our state mandated end of course test for Algebra 1, the kids who take it in 8th grade (15-20% of kids in my district) or 7th grade (1-2% of kids) significantly outperform those who take it in high school. Clearly they aren't harmed by taking the class earlier. They do well because in general they are better at math than kids who take it later.