Algebra I is a crucial course for middle and high school students for successful STEM related coursework. A key issue is whether students should take Algebra I in 8th versus 9th grade. Large-scale policy studies show conflicting results, and there are few (particularly longitudinal) individual difference studies. Here, 53 students were assessed in 6th grade on cognitive, numerosity, and mathematical skills, and then followed; 26 students took Algebra I in 8th grade, and the other 27 in 9th grade. Comparisons between groups at grade 6 revealed gaps in some (but not all) cognitive skills and on mathematical competencies, but not on numerosity. By Algebra I, gaps in cognitive skills diminished, but gaps in mathematical skills remained constant. Gaps in algebra skills were also apparent, despite the age difference between groups. Results suggest that the additional year of instruction was not optimally tuned to pave the way for strong Algebra I performance. [This is the in press version of an article published in "The Journal of Educational Research."]