Very rarely will you see a 9th grader on varsity make a huge difference and they are usually up there to fill in on defense somewhere. Now with that said there is your occasional stud, but more often then not they don't get a lot of reps during the season unless they bounce between JV and Varsity. My 2021 son played JV all year last year and bounced up to varsity a few times to fill in as backup catcher.
He's been told that he'll be on varsity as a sophomore, which is all cool to say and everything; but I'm not sure how many games he'll play...but he made a huge jump from Spring => Summer and into Fall so he is read to play. And I think that is the key for underclassman... how much time have they dedicated to getting better? Even with playing multiple sports they still have to find time to get better at baseball no matter what grade they are in.