Age restrictions to handguns?

Why must an individual be 21 years old to purchase a handgun yet rifles and shotguns may be purchased at 18? This goes for ammunition respectively as well. I understand the concealment of handguns is a lot more feasible, however, if I was to actually going to use the weapon to kill someone it would be with a rifle from a distance, and if a killing spree was the intent, a shotgun would be highly more effective. So why the age restriction? They should all be either 18 or 21