This is, of course, a matter of opinion. My own view is that juveniles of this age should not be eligible for capital punishment.
My first reason for saying this is that I generally oppose the death penalty for adults as well as for juveniles. I oppose the death penalty because I do not believe that it is administered fairly. The death penalty is given to some people, but not to others, on the basis of very subjective decisions made by judges, prosecutors, and juries. If we could know that people with equal amounts of moral culpability always got the same punishment, I would be more likely to accept the death penalty. As it is, it seems that we mete it out on an arbitrary basis. Because I do not think that the death penalty is administered fairly, I do not think that juveniles should be executed.
But even if we argue that the death penalty is appropriate for adults, I still would not give it to anyone under 18. In fact, I would prefer not to give it to anyone under 21. If we say that a person is too immature to handle alcohol, they are surely too immature to be morally culpable enough to deserve to die. This goes even more for a person who is not allowed to vote or to buy a lottery ticket because they are not mature enough. If a person is not mature enough to be allowed to make those decisions, we surely cannot claim that they are mature enough to make decisions that should determine whether they live or die.
For these reasons, I do not believe that the death penalty should be applied to juveniles between the ages of 16 and 18 or, preferably, 21.