Owners and/or coaches. When evaluating your team either after tryouts or summer practice. You have alot of kids who are at or near the same skill level but age is a wide range though. Lets use 20 kids for easy math. Say 15 of these kids are ages 8-14, the rest 15-18. Where do you put your focus as an owner/coach? ( Jr., Sr. open) If you go jr. then you have to have kids/parents that will cross compete so the other kids can still be on a team. or go sr. open and now you focus on a team that seems to me (by reading posts on this site) looses kids to high school cheer every year.
Based only on your information, the team would go Senior. I would talk it over with the kids first, then the parents. A lot of kids/parents think Senior team means best team and not age division, so I would want to make that clear to them unless I wanted to have problems next year with a junior kid/parent who felt they deserved to be on a Senior team because they were on one last year.