Do they actually have any proof that 12-17 year olds are the largest porn viewing demographic? I don't believe that at all. Make it 16-21 and I'll easily believe it.
I suspect they picked 12 as a lower bound, since reaching down before the typical onset of puberty would not be credible, and they wanted folks to be shocked by 'children' viewing porn, then picked the narrowest demographic age-band widths so that the one with 12 at the bottom would be the largest.
Either that or they started at 17, since 18 is age-of-majority, and picked the broadest age-band widths that gave a group of minors as the largest group
of porn viewers.
Either way, I suspect a dishonest research procedure was used to give the
desired result for shock-effect.