At what point does it become unacceptable for a mentor to design/build the robot

[Taylor]

This is a struggle - one that many old farts still have.
This is my eighth season in FRC, and it made me cringe when on the first Monday the students were saying, "I like Taylor's idea. Let's go with Taylor's idea."
The thing was, the way we chose to approach this game was decided by the team. The strategies we'll employ were decided by the team. And when I say 'team' I mean students - I only acted as a scribe on the chalkboard. "Taylor's idea" was simply a way to realize that strategy.
Since then, it has been refined and massaged and manipulated, mostly by students, into a workable solution.
I suppose my tl;dr is this: Ideas are ideas. They're not property. Once they're out, they're not owned by anybody. So if you feel your mentors are hijacking your team, make a concerted effort to stick your nose in, work shoulder-to-shoulder, make yourself a part of the process. If they deny you that, demand an explanation.