For my DD’s school, seniors are required to be on Varsity. That said, being a senior doesn’t guarantee playing time. The 3 seniors on Varsity last year didn’t get much, which was a reflection for their skill level within the team.
I don't have any set rules, but I won't put a senior on JV. I believe JV is for development to get players ready for Varsity. I did play a senior in a JV game last year just to get her some ABs since she didn't play much and the JV team was down a couple players, but I don't see a lot of value in a full-time JV senior.