There's always room here for one's opinions. But, there's also room for civil discussion of them as well. I don't agree that for every Audie Murphy there is a Bergdahl or a Manning, not at all. That is simply not true. Courage such as Audie Murphy displayed is indeed rare, and it is partly due to the circumstances in which he found himself and of course, how he reacted. How he reacted is to a large degree how he was trained. The pitiful scum known as Bergdahl and Manning are not so much rare as they are aberrations. I also question your responses on the military as it relates to maturity and responsibility. Are you basing this assumption on personal experience or ...?
In addition, a young person who has successfully been through Basic Training, or Boot Camp, as the case may be, has shown through his or her actions they have at least some level of maturity and the ability to make at least some decent decisions. They have also demonstrated some basic proficiency and safety with weapons. Their peers, on the other hand, may not have had the same life experiences as of yet, particularly appropriate in this discussion as it pertains to the handling of weapons. Now, I don't think we need to raise the age to 21 in order to purchase a weapon, but if it is done by legislative action and if it becomes the law of the land, then yes, I'd like to see the caveat applied to let those under the age of 21 with basic or boot camp behind them have that right.
