Private educator here:
We work with our students in a way that keeps them at their best throughout their time with us. So, we help 12-year-olds and older reach their potential for their level of brain development and maturity and up that over time as they develop.
While it’s never too late to prepare for stronger results, we recommend that students build mastery over time, starting in middle school, if not before.
This need not be stressful. Forget flashcards, canned test prep programs, official test dates, etc. If you can find a pro who is professionally trained as a teacher and who personally earned very high scores on their first tries, ask them to help you reach your limits over an extended period so you’re always at your best.
This approach will help you get higher grades and walk into standardized tests without special prep and earn high scores dependably.
I did this with my own son as I homeschooled him and worked with my own private students for years to allow me to do that. I am certified to teach in two states currently and have two degrees and lots of experience working with various kinds of students. His scores on his first tries after his junior year were each the highest possible score, without specific prep for the tests.
It was so unstressful for him that he too his ACT, SAT, and all three SAT II tests that he took (all STEM) while he had food poisoning, earning ACT 36/36, SAT 2400/2400 (same as today’s 1600/1600), and SAT II’s all 800’s. However, one of his SAT II tests was taken in January of his senior year. He also got all 5’s on his STEM APs, and 4’s on his Language and Literature APs.
So, when should you start? Always be at your best at any given time. Failing that, start early and build your mastery over time. Last-minute test prep can be effective by significantly raising your scores IF you have highly skilled and available teachers. That’s hard to find, no matter what the salespeople (aka “directors”) say.
Don’t take chances by waiting. In fact, don’t even think about waiting. Many parents believe that taking a test prep course immediately before a test is smart “so they don’t forget anything.” That perspective assumes that students will memorize facts and then do a brain dump during the test. No, no, no! Students need to build deep mastery over time. Very little memorization is helpful. Skill and overall knowledge are far more important. Yes, memorize a few formulas, but know that highly competent students may not need a memorized formula to be able to accurately answer a question. They understand so much that they can figure it out with what they know. That’s true for my son. He’d never been one to memorize as, like me, he wants to understand instead. So, he’s done that and earned those perfect scores that way. For most people, it’s important to memorize a few formulas but also have very solid mastery built both on understanding and lots of practice. That takes time.