I don't think he'll get any less time because of his relative age. We allow and accept people at age 18 to enter the military, and they are considered full adults, trained in the use of lethal weaponry, etc. He may have been influenced by his brother, but he was an adult.

I also do not "get" the huge debate over whether he should be read his Miranda rights or not, as he graduated high school and was in college, so I am sure, having spent much of his life in America, watching TV and going through school, he could probably recite his Miranda rights in his sleep. That groups are battling about this is, in my opinion, just dumb. If he survives, he will have to face the consequences of what he has done.