You have to be 21 to buy a handgun in the United States. The gun control act of 1968 maintains that it is unlawful for a gun dealer to vend a handgun to anyone under the age of 21. However, the agency's regulations only apply to federally licensed firearms dealers, not to nonprofessional private sellers.
Some states vary regarding gun laws. You can't buy from a gun dealer if you are under 21. However, you can buy from a private seller in many states, but not all states. An 18-20-year-old cannot obtain a concealed carry license in any state because the mandatory age is set at 21.
No, an 18-year old will not be able to buy a handgun in the United States but he would be able to buy rifles and other guns. Take note that there are some differences with the laws and the regulations depending on the state that the 18-year old is from.
The reason why the handgun can only be acquired by those who are 21 years of age and above is because a lot of crimes that occur are still handgun related. This means that people use handguns in order to do crimes. With all of the recent shootings that have occurred, it is best if buying guns can be regulated further. It is not only handguns that can be used to make a crime happen.