if a student takes SAT subject tests in 8th and 9th grade, will the result still be valid during senior year? Can they send the result when available or hold till application year?
For 9th grade, yes.
For 8th grade, yes, but you have to request by the end of that academic year for the College Board not to remove those scores. Otherwise, they are wiped out.
https://collegereadiness.collegeboard.org/sat/register/special-circumstances/younger-students
Now, whether a college will accept old scores is a different question. Most will, but you should check, as I don’t believe there is a comprehensive list of colleges that do or don’t.