Maybe the men you know. The average age of soldiers in WW2 was 26 and Vietnam 22. That means a whole lot of men under the age of 21 using guns to help defeat Nazi Germany and Imperial Japan, and many even younger than 18 who forged their way into combat after Pearl Harbor.  Why do you trust a 20 year old to use guns in military combat yet not to choose whether to smoke, drink, or own one a personal firearm?