Great topic. I'm no HS coach and my 2017 son is a freshman at college, but that won't keep me from chiming in.
I have mixed feelings on kids playing up in HS. First off, a middle school kid would have to be amazing (a true no-doubt stud) to make, let alone play, let alone start on a HS V team (and that team would probably have to stink on ice too).
I'm not a fan of Fr playing V unless they play a lot. Not necessarily start, but they should be solid contributors, otherwise let them be leaders and get reps on JV.
Of course, you play the best 9 at that time. However, if two players are truly very close in what they can do NOW, and they are both on the roster, I generally support giving the upper classman a shot. Is this entitlement? Perhaps a little, but I'd call it: true push goes to the older player. Again, if an underclassman is better, he gets the opportunity.
As far as making the roster. In general, IMHO, junior year is the time to cut players, when cuts are needed, who have not developed enough to merit the spot. I'm not a fan of HS rosters with 30+ kids, and I've seen them. But if a hard-working kid sticks with the program and is a senior, AND there is room on the roster, let them stay and ride the bench. Just be honest about their role.
My son was a year behind a catcher whose uncle and brother were the starting catcher for our modest sized HS in NoVA. I was told by other parents that my son would play JV and be the back up until the other kid graduated. Half-way through my son's Soph. year he got the nod and the other player went to DH and 1B. And folded like a tent, batted like 0.100, and was livid about the demotion (I heard). He quit baseball and did not play his senior year, which did hurt the team.