A senior was recently busted at my local high school this past semester; Maybe you even brought this up because of recent events like this. I have been thinking the same thoughts as you as well. I see no oddety or un-moral characteristics towards a kid 'under age' to be looking at 'under age' pornography. Yeah maybe it could lead to a future addiction or problems, but then that would create pre-facto based laws which are obviously unjust....maybe they could be treated as being in need of some kind of therapy, but legally I think it dangles on extremely fine lines.
I dunno. It's a pretty funny and odd subject in my mind....