I don't believe in conscripting anyone to fight, but such positioning creates some military weakness. If your'e going to have a conscription law then it must be applied equally.
If you have an age of adulthood, wherever you choose to position that then it should apply to conscription... you cannot argue people shouldn't be allowed to occupy positions in Government/vote until X age, but allow or compel them to serve in a military over which they have no direct means of redress.
I suspect the upper age should also apply, there are some kick-ass 61 year olds and some super fit 70 somethings who could still perform military functions. Perhaps there should be an upper age limit on voting. 18 years earlier than the average age of death in a country.
Only those in the middle get to vote because they're the ones who get to go to war. :o/