This article intrigued my interest when it stated…

“…studies have shown that studentsâ€™ knowledge of sixth grade fractions is a good predictor of their math achievement in high school. This is largely because a deep understanding of fractions plays out in algebra.”

https://ww2.kqed.org/mindshift/2016/11/21/what-neuroscience-can-tell-us-about-making-fractions-stick/