My son joined a troop that has senior scouts (youth) sit on the non-Eagle boards of review. Officially speaking, I know this is wrong. The "Advancement committee policy and procedures" book clearly states that BOR should be comprised of committee members. (
Thanks Ask Andy for all the great training!) I fully believe we should be 100% compliant with the official BSA policies and procedures. Unfortunately, I am not in a position to change our troop practices.
Practically speaking though, what are the positives and negatives (other than violating BSA policy) about having a youth on the BOR? 
I can see some positives in that it promotes taking leadership seriously in the troop. It could be a point of pride for those youth who help run a BOR. It could also be a negative because it could make the scout under review be less open and defeating the quality control role of the BOR. For the same reasons that SMs and ASMs are not allowed to be on the BOR, it seems even more so that scouts should not be allowed on the BOR.
Is there a critical reason to push the envelope now to get this changed?