The government may like such a double standard because it makes their job easier, but as citizens we are supposed to make the government do what is right, not what is easy.
If you have to fight, then might makes right. It's that simple. Veterans learn a great deal of maturity. In our history, in the organisation of militia units many of the officers were elected by their men.
Is there any principled reason we should not let 16 year old's enlist with parental permission?
I'm sure there are practical reasons this might not be a good idea...younger recruits might be too weak to fight, too hard to train, etc. But if those obstacles were not present, and assuming there is no national emergency, do you believe it would be acceptable for the government to allow under-17 minors to enlist in the military?