Though one can take the test anytime irrespective of the grade/class and age. Mostly, it is recommended that you take up this test once you are through with your Calss11.
There are no prescribed minimum marks for appearing in SAT. Also, the SAT scores are valid for five years, where most of the colleges/ universities accept up to five years since you have taken up the exam.
As per the experts, the best time to start preparation for SAT exams is in class 11, though exam can even be taken after class 12 , based on any recognized Indian University. It is important to keep college application deadline in mind, as admission decisions are largely dependent on SAT Scores, along with GPA, Transcript, Recommendations and more.