I think you can draw the line where people plan and act like normally an adult is supposed to do. Your biological age is of course linked to your mental age, but some people act like adults at age 16 and some people are more compared to 16 year olds at age 18. Of course, the same person at 14 or at 18 will act differently, but the fact is that some countries treat a juvenile as a full adult if it can ascerted that its behaviour is in cordordance with adult behaviour, the reverse is also possible.
In this case, the planning, the amount of killings and the way the killings were performed certainly indicate that he knew what he was doing. I'm not familiar with US crimininal and juveline laws, but here he would certainly by fully trialed as an adult with subsequent possible punishments.
This is something that hasnt been said in this thread yet (if it has ive missed it). Plenty of times adults are ruled to be mentally incompetent and are sentenced accordingly. We arent talking about someone who replicated a wrestling move and accidentally killed someone. We are talking about someone that helped murder over a dozen people over months.