20th century seems pretty irrelevant.
Not a bad idea at all.
I think the argument for that one was that it was unfair for people to be drafted and yet not be able to vote. I would think a solution would be to require that military service be a basis for full adulthood for the duration of service, or permanently following a successful term (e.g. if the normal age of adulthood is 21, and someone joins the military at 18 for a two-year term, that person would remain an adult at age 20; if someone joined at 18 and quit at 19, the person would lose adult status).