You are too old to drive a FIRST Robot. *
I think this is a great idea that has a lot of potential.
As others have pointed out, other than a few very specific rules, there is a choose your own adventure on design, build, programming, scouting with a lot of different systems that work for different people.
I will say that in general, some age separation between a primarily HS program and mentor program is beneficial from a sorting out social situation stuff. While not the majority, there are frequently issues related to having an under 21 age "mentor’. There are a lot of good examples as well of programs that make this work, and specific situations where problems have arose.
Some example “problems”:
Young mentor transition from skilled student to mentor may want to do “too much” relative to the teams standard interaction.
Young mentor may have authority/respect issues.
Dating or Romantic Involvement Policy. I think before venturing into such an area, making sure you have good policy to cover your team as well as a strong understanding from individuals involved is paramount to avoiding difficult situations. Some folks are more flirtatious than others and when this is in a “mentor vs student” dealing it can become problematic. Once it is problematic, it can escalate to something pretty serious pretty quickly.
While youth is wasted on the young, in FRC knowledge can be wasted on those a bit older. Often those with the most enthusiasm, energy, and sometimes FRC specific knowledge are those that just graduated from HS and college. Unfortunately, they tend to not have a lot of vacation time or work/school schedule flexibility. They also tend to have less general $$ for paying for travel and hotels. Those that have more engineering experience and vacation accrual tend to have family commitments that can make participating difficult.
Using team funds to “sponsor” a non-HS student can get tricky for some teams. Expecting mentors to pay their way can be hard on a college or 1st year employee who is already using most of their vacation to go to a regional and possibly World Championship. Paying for 1 mentors room and board without paying for others can lead to internal leadership struggles/politics.
Most of these can be mitigated with clear team policy, people willing to respect those policies, and/or cool heads when expectations are not met. As a leader, you can set standards and team policy and educate those involved. For the other two parts, you can only do your part by modeling good behavior and being as good a communicator as you can be.
*As a mentor when Mike was a student, I believe I can take a poke at him…