I know with wrestling, 10 years ago or so when my son was 7th and 8th grade, he would face 8th graders from Ohio, North Carolina and such that wrestled HS level as 8th graders at national duals. They were at a different speed. I know south Dakota you can go at 7th grade, MN 8th grade. Ohio at the time was 8th graders could wrestle JV but not varsity.
Also, how many juniors or seniors do you see wrestling the bottom couple weights? Not many. So it would be no different than that 9th grader wrestling a senior at 150.
Basketball, wrestling, baseball, track, I don't see big issues. Football.......JV maybe but Varsity football I'm not in favor. With my kid playing for a small school, I saw them go against too many freshmen who were not ready for Varsity and don't want them throwing an 8th grader out there.