SAT testing takes place while in high school, so test taking starts at age 15, but it's probably mostly 16 and 17 year olds who take the test I'd think.
Or significantly earlier (e.g., the John Hopkins University Center for Academically Talented Youth uses it as one of the options for qualifying exams for their programs students starting in Grade 7.)
Or significantly earlier (e.g., the John Hopkins University Center for Academically Talented Youth uses it as one of the options for qualifying exams for their programs students starting in Grade 7.)