For starters, not everyone is of military age. Does that disqualify them from having an opinion on matters of nation and international security?
Additionally, even if they aren't willing to enlist themselves, that shouldn't disqualify them from having an opinion on this. They pay taxes that fund the military and other agencies for the purpose of national security. Countries have militaries for exactly these purposes. Citizens enlist in the military with full knowledge that they could be called to battle one day. Do you think the military should only engage in war if every enlisted member agrees to do so? No other input should be taken?
Additionally, even if they aren't willing to enlist themselves, that shouldn't disqualify them from having an opinion on this. They pay taxes that fund the military and other agencies for the purpose of national security. Countries have militaries for exactly these purposes. Citizens enlist in the military with full knowledge that they could be called to battle one day. Do you think the military should only engage in war if every enlisted member agrees to do so? No other input should be taken?