I agree with you that a 15 would have the knowledge necessary, but not always the means to change.  If they are still living in their parents' house on their parents' dime and subject to fatassery, they may not be able to arrange their own diet.  I know many could just eat less of whatever crap their fat parents buy, but I knew far too many parents, even at that age, who had a 'finish everything or you can't leave the table'  rule, and would basically bully their children into overeating.   I think that until the kid is able to get a job or leave the family situation, the parents are at fault.