The SAT is basically an aptitude test that covers all of your grade school learning. So when should you start studying? As early as possible!!! You can start at any age and any grade once you have mastered the basic mathematical and English concepts.
Will you get the problems right if you start very young? No!! But you will get used to seeing the problems. Ideally the math section could be started with some degree of success after PreAlgebra concepts are mastered. English is a little tougher. The reading is not easy, but you can read but you can read a few sentences every day.
Look up the words you don’t know. Memorize these words and start using them in everyday life. By the time you are ready to really study for the SAT, you will be way ahead of the curve.
I have had students show up in twelfth grade right before graduation. One student comes to mind. This particular student was trying to qualify for a scholarship to a very prestigious and expensive voice program. Her dream was to be a singer. This was one bright girl and was already a great dancing student. Trouble was, she started with me 2 months before graduation. As a dancer, she had to attend several competitions. She worked a part time job and helped with her family’s business. She also had everything that comes with being a senior going on. She had prom, senior dance, senior awards, senior pictures and the list goes on and on and on. This smart girl worked really hard when she was with me. However, she didn’t have time to practice on a consistent basis on her own.
I don’t know if she met her goal. Had she started earlier though, I know, nothing would have stopped her from achieving her goal.
The key is to start early and give yourself the gift of time.