If those issues were brought up in the same context of 'if they're old enough to fight they're old enough to....' then I'll stick with the idea that once they actually put themselves in the position of having the responsibility then they can have the privilege.
Are there inconsistencies that I don't understand? Sure. Why can an 18 year old buy .22 ammo for a rifle but not for a pistol?
IMO by denying youths alcohol until 18, 19, or 21 we provide incentives for abuse by way of the Forbidden Fruit instead of teaching them responsibility as the mature.
Again, imo privileges go to those who step up to the potential not just because of the potential.
I do understand where you are coming from. While I don't agree with your approach, you seem consistent in how you would assign the rights and responsibilities of adulthood. That's what is most important to me.