The earlier the better, but you can probably start in 10th grade to prepare for it. I think the best way is for your child to either have a tutor or take prep courses (Kaplan is a good place). You can also get the prep books such as Barron's, Kaplan, or Princeton Review to name a few. There are also some vocabulary and math flash cards out there that you can use to help your child prepare. I think the more practice that your child gets the more familiar they will be for the actual exam. You can get some practice exams from the internet to help your child as well. Good luck.