Nobody can tell you why a kid is useing this site or why they would register fact of the matter is there are kids useing this site regardless of what you may think and the section would be very simple to access. For it to work strict rules would have to be enforced and it be monitored for random XXX images and topics, chat depending what rules are in place.
what we can express on the site now can be classed as adult talk and really there is no difference the odd topic may change but not much i'm not a staff member here this is just my opinion. i agree it would be a nice idea to have an adult section but these are the things i would put into theory you could also just say who cares and go ahead i'm sure the staff here may show a little concern.
As you may have seen some torrent sites have 18+ areas but that's their choice and i like the idea.