I would like to raise the age but you are wrong on both counts. GIs between the ages of 17 to 21 have a lot of responsibility and for some of them the judgement required of them will be higher than what they encounter in their civilian jobs during the rest of their life.
The draft ended almost two years after the voting age change.
I am not wrong. If the voting age had been changed only for military veterans, then you would have a point. But that was not the goal.