My problem is that if you were a human living in the time of Starship Troopers, the only way you could vote against an unjust war would be to submit to a national service process where you might be forced to fight in the very war that you judged to be unjust.
On page 143 of Starship Troopers in a discussion of a 'History and Moral Philosophy' (mandatory) class in OCS.
Throughout history men have labored to place the sovereign franchise in the hands that would guard it well and use it wisely, for the benefit of all. [snip failed examples]Under our system every voter and officeholder is a man who has demonstrated through voluntary and difficult service that he places the welfare of the group ahead of personal advantage
Assuming that you were old, wise and responsible enough to vote against an unjust war, you would have volunteered at 18 for a couple of years of national service, maybe or maybe not a war, but difficult and dangerous nevertheless. You are discharged and now you can vote. If at 18 you chose to chase the opposite sex and big bucks no problem, you just don't get a vote on any war just or not. You still can make the big bucks supplying the soldiers, you just can't vote on whether they should be spending the supplies in that way."
An interesting side note is that you can't vote while you are involved in national service either. I do not think Heinlein thought this one through. I would think that citizenship would be bestowed after 2 years anyway. One has paid one's dues and if one is willing to continue as a career person in a service organization I would think the vote input would be even more valuable.