Eighteen yr olds may have the intellectual maturity to vote intelligently, but by and large, teenagers don't have the emotional maturity to responsibly drink or carry concealed weapons.
So how do you feel about enlisting, owning long guns, and getting married? I'm twenty, I'm enlisted in the USMC, married, and I have a twelve gauge to defend myself and my wife. All legally. I can't be trusted to carry a concealed handgun, but I could own a .50 BMG or an M1A without a problem? What about driving? In many ways more difficult to do safely than shooting.
And why do we need artificial, randomly chosen age limits? The drinking age in Ireland is 16 IIRC. I'm always amazed when someone who complains that the government would prohibit them from carrying because they
might do something bad turns around and says that we should ban people of a certain age from carrying because they
might use it irresponsibly. Why don't we prohibit and punish
behaviour?
You say people under a certain random age are more likely to be irresponsible, and thus should be prohibited from exercising the fundamental right of self-defence. Statistically speaking, black men who live in inner-city areas are more likely to use guns criminally. Shall we prohibit them from owning guns?
The only result of artificial age limits is telling young people that they aren't responsible for their own action until they reach a certain age. Oddly enough, they sometimes believe this.