Well there's no standard curriculum in the US, but based on my personal experience in Virginia and Maryland, matrix math in Algebra I is common. The problem is

Well there's no standard curriculum in the US, but based on my personal experience in Virginia and Maryland, matrix math in Algebra I is common. The problem is that that's never built upon and it seems many math teachers don't have a firm grasp on why matrix math is useful, much less the ability to explain its utility to typical ninth graders. I encountered vectors later in Physics (which was the most popular class to take as the one required science elective in my high school) and in one of the AP Calculus classes (which a large majority never take) Matrices were revisted in Algebra II, with nothing new relative to Algebra I except that solving the matrix equations we were given required using Algebra II-level techniques on scalars. I guess a bit of spaced repetition might mean that more people remember it, but overall most students just forgot about matrices right after the test.