What you seem to be implying is a return to a form of National Service (I believe you yanks called it the draft or draught or some such name).
To blindly issue a graduate with an armament without suitable training in it's use and upkeep is the height of irresponsibility.
Apart from that, it has long been proven that the (draft or draught) was notoriously unfair, was riddled with corruption and was economically unviable.
It has been resoned that a volunteer fighting force is better than an involuntary fighting force.
You may not be familiar with certain aspects of American history.
In our early days, we had almost no standing army. The closest thing we had was a "Select Militia" much like the modern National Guard or Army Reserve. Most military force was in the "unorganized militia", which basically meant every man capable of bearing arms. In most cases they brought their own muskets or rifles. This was the backbone of American national defense for quite a long time.
Korimyr is, imo, alluding to that original use of "militia", in that every American of fighting age is technically part of the "unorganized militia", and subject to the call to national defense in time of need. In acknowlegement of that civil duty, he suggests that some basic ROTC type training and the possession of a suitable weapon should be considered a civic responsibility. I agree with him.
This is still no small factor in our national security. Admiral Yamamoto, talking with the Japanese high command about starting a war with the USA, advised against invading mainland America after Pearl Harbor. His reason? "Behind every blade of grass would be a rifleman." He had been educated in America.
In reaction to your assertion that issuing an 18yo with a rifle, without some sort of extensive training, is some kind of irresponsible silliness, reflect on the fact that the USA is a culture of arms; many of us grow up from childhood hunting and target shooting and handling weapons. An 18yo in the US may purchase a rifle or shotgun on his own, and is subject to no restrictions other than not being able to buy a handgun until he is 21. In many or most states, there are no particular requirements other than a clean record... yet gun accidents have dramatically declined for over a century and are a minescule fraction of other accidents (like automobile fatalities).
Frankly letting a 16yo drive a car isn't really any more likely to end in tragedy than letting him handle firearms... less so, statistically.
I refer you to my post above, where I discuss children I have trained in the use of firearms, and my own son's upbringing very similar to my own... nor is my case at all unique, a good many American young men are still brought up to learn survival skills, including firearm skills, as a simple fact of growing up.
I daresay I could easily introduce you to a couple dozen teenagers who are as adept with firearms as the average soldier... yet they are vanishingly unlikely to criminally abuse a firearm because of how they were brought up and taught a strict code of morality, ethics and honor.
Your concerns might be more valid in a European country, where the average 40 year old has never touched or fired a gun. In America, it is not unusual for a rural child to shoot his first deer at 13 or 14.