Sixth graders certainly will be able to understand the concepts. Many of the teaching options especially for younger students (targeted to 7th graders) will be very appropriate for 6th graders.
However, 6th graders have different developmental needs from those who are only a year or two older. Many of them may have difficulty dealing with some of the abstract ideas that their older peers can begin to handle. Teachers who are using StudentLifeBibleStudy for 6th graders may need to make adaptations to fit the lessons to their class.