It's funny that no one ever takes a serious look at Bob Heinlein's solution:

The military is strictly volunteer. And service in it is arduous and unpleasant. But you can walk into the CO's office and quit anytime you want.

There are alternative forms of national service for those who are unable to serve in the military for valid reasons. Some means of service is available for anyone, regardless of physical or mental limitations, provided only that the person has the mental ability to grasp the concept and the emotional control to accept orders. But all the alternatives are also arduous and unpleasant. And you can quit any of them at any time.

You have all the rights of citizenship whether you serve or not - except one.

When you have finished a minimum term of national service - then you get to vote.

Peter.