You may be aware that NRA lost a lawsuit in Texas challenging the state law that requires you to be 21 to obtain a concealed handgun license. Shall Not Be Questioned has some decision about it here. This follows a similar decision by the same federal judge who upheld a federal law that prohibits sales of handguns by dealers to those under 21. (You can buy a long gun from a gun dealer if you are 18, but not a handgun.)
Keep in mind that Judge Cummings ruled in our favor in USA v. Emerson (N.D.Tex. 1999), so he is not a stick in the mud antigunner. Indeed, when he ruled in our favor in 1999 (and cited my second book), he was doing something a bit courageous.
Federal and state laws discriminate against 18-20 year olds concerning handguns. Keep in mind that historically, 21 was the legal age for adulthood in the U.S. When I was young, the laws of most states distinguished juveniles (those under 18) from minors (those under 21). You were not a full adult until you reached 21.
I admit that there are many 18 year olds who are quite responsible and mature, just as there are some 25 year olds who are not. These age limits are arbitrary, just like those used for sexual consent, driving, and a zillion other areas where our laws make distinctions. Simply because they are arbitrary, and therefore frequently wrong, does not make them automatically suspect or erroneous.