First, yeah, you gotta list all your courses. But I think adcoms really do look at people's transcripts carefully. One C won't make or break anyone's gpa. (Of course someone is now going to post about how a C WILL lower the gpa--yes of course I know it will but one single C cannot ruin an otherwise good record, and if the record's bad overall, the same C won't stand out.) When they see that you were taking this class at age 16, I'm sure they will be quite impressed. Seriously, I would not risk omitting this under any circumstances. It wouldn't be worth it. When you start filling out your AMCAS form, you will realize how wide open your life becomes to scrutiny. Hiding things just isn't smart.
Second--hey, I'm just wondering if anyone has ever taken an actual look at the 1974 Educational Privacy Act or whatever it's officially called. I wonder if it has any stipulations regarding academic records of minors. Personally I think it is unfair for some people's high school records to be required in this process, just because they took college classes. If you'd taken the same class at age 13 or 14, could AMCAS require a transcript then too? How legal is that?