1. No one said they were "stupid".
No, they didn't use those exact words. However, they did say that they were too unprepared to make such life choices for themselves.
2. They wouldn't be choosing who leads the military. They would cast one vote out of 50 million for one of two options. It's not really democracy, it's a facade of democracy, and it's a joke to call yourself powerful because of it.
However, they would be suddenly prepared to make a correct selection, to choose who should lead what they are too unprepared to be part of. This is preposterous.
3. The things required to fight in the military and the things required to vote are completely different. You can't lump one into "larger responsibility" and the other into "smaller responsiblity". That's exactly where libertarians go wrong, believing that there's some universally consistent approach to everything whenever there isn't in reality.
However, I can suggest that if they are unprepared to make choices for themselves they are wholly unprepared to make choices for others, such as who will lead those they cannot be part of because they are 'unprepared' for such.
You don't believe that the voting age should be raised to 21, Damo. I don't know why you're making such an argument other than to be an ass.
No, I don't believe that the voting age should be raised, I also don't believe that the age to join the military should be raised. I am pointing out the fallacies in their argument. They are totally unprepared to make long-term choices for their own lives yet are supposed to be able to make choices for mine? I don't think so.
Those making this argument are attempting to have it both ways. They are either unprepared to make choices for themselves and their own lives and thus unprepared to take part in making decisions for others, or since they are prepared to make long-term decisions concerning others they are also prepared to make them for themselves.
I suggest that they are both. Considering I put myself into that same decision making process at that age, and all the friends that I was in with also did, I'm reasonably sure they were well aware of the implications of their service.