I trust the same kid at 12 moreso than the same kid at 7, 8 or 9 years old. I can't imagine giving my toddler a gun and having the kid tote a gun and trust they will not fire in somebody's direction.
Yes, there are laws that are broken routinely which are hard to enforce, but at least the previous rule sure seemed to stop kids in diapers carrying guns in the woods on deer drives. Since there is no minimum age law and the "mentor" can now carry a gun, a warden stopping guys after a drive coming out of the woods cannot prove a kid was not being mentored. Before this change, if a kid 9 years old came stumbling out of the woods with a gun, there had better been an adult without a gun in the group.