As our team is based at Iowa State University, quite a few of our members continue onto college there. We have an extremely strict policy of requiring alumni to take a year off before returning to help mentor our team. I took like 2 years off, and even then I still mentored a couple seniors that were freshmen when I was on the team. We also push students to ensure they keep up with their studies their freshman year so they don't drop out.
In my studies for Emergency & Disaster Management, one of my teachers gave me some good advice about what the controllers/evaluators do (that are very similar to mentors) in that they treat the participants/students like they are on a ride at Disney World and being that middle guiding track. Ensuring they stay within the boundaries (getting them back to the middle if need be) and never veer off that single middle track too far.
I mentored Neutrino last year and looking back, I could have sat on my hands a bit more (and plan to attempt to do the same -- which should be relatively easy as I'm event director for an FTC Tourney the next day
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As other people have said, "Mentors guide & teach while students do, learn, and the senior students teach." If you think you might be overstepping your boundaries, take a step back and observe for awhile. Let the students come to you for help. As a mentor, you really shouldn't be touching any of the parts of the robot. Let the students do it!