What is the average SAT score that a 7th grader would get if all 7th graders took the SAT? This is distinct from the average SAT score of 7th graders who decide to take the SAT.
The average 7th grade score (for all 7th graders) on the new 1600-scaled SAT would be 600. I calculated this by using Fred's analysis here (that states an average 7th grade section score is 300) and multiplying it by 2.
You are correct that the average SAT score of a 7th grader who does decide to take the SAT -- which is an empirically measurable score versus the inferred number above -- is much higher at about 1000. This is because 7th graders who take the SAT are usually already scoring at a higher-than-average level (since many of the students who take it do so in order to qualify for a talent search program like Duke TIP or CTY).