Not everyone can serve in the military
Bad eyes, some sort of injury, too short, too tall, feet too big. So they should be prevented from voting?
I can see raising the voting age to 21 or better 28. When they complain that they can buy beer at 21 but not vote then raise the drinking age.
Having military service as a requirement to vote would be as heinous as letting 16 year old kids vote.
I add the military for the younger group for the following reason. Those willing to risk their life to defend this country, or whatever political posturing, should have say in those who would send them into harms way.
If they can hold a civie job, in most cases, they can hold a military job. Most military jobs are in support, not frontline. Although still, a small percentage in no way can hold military jobs like you say.