Really those points (voting/drinking beer) have nothing to do with each other. Drinking laws aren't addressed by the Constitution and any linking of them is just an emotional red-herring not a logic based argument. I am not saying you shouldn't think it but rather that there is no inherent link beside you deciding there is one.
I have been out of school for many (many) years, bit I recall there was a lot of evidence that suggests the most emotion driven and easily manipulated voting group are under 21s, so I suppose one could conclude extending the right to vote to them was a bad decision or at least fraught with peril. Also, regarding military service, being eligible to serve is not the same as having served. I can see the utility of a tiered system with some age being the base for voting but with actual (not theoretical) military service causing there to be an "early entry" system?
Now as to gun ownership I am ambivalent. Without a doubt, young men in the post-pubescent group (16-19/20) are the ones most prone (or at least most arrested per FBI statistics) to fighting, violent behavior, gang activity, etc. BUT anyone who could be called up for militia duty should be able to "keep and bear arms".
I appreciate your opinion.
I agree there is an element of emotion involved. Any topic involving our rights is bound to be. But I would argue that an 18-20 year old doesn't instantaneously become more responsible the moment he graduates from basic training. It doesn't take much looking to find that indeed, young soldiers/Marines are just as prove to stupidity as the general population. It might be less only by virtue of how tightly controlled their lives are by the institution.
I disagree with the idea that we should create a two tiered system whereby those who have enlisted in the military have more rights than their peers back home. They enlisted to defend all of our rights, and it strikes me as unfair that veterans should get preferential treatment under the law in this manner.