Purchase of handguns is an age of license issue not an age of majority. The state (in the generic sense) can permit activities to occur at certain ages which are different than a general age of majority. You can get a DL at 16, or a hunting license at 18, or get married at 18, or drink at 21 etc. The age of Majority means that you are responsible for your own decisions. So at the age of majority you can sign contracts that legally bind you without parental consent. A person underage can marry, drive, join the service etc. but it requires the consent of the parent or other guardian to do so. At least this is what I read on Wiki
Interesting DL procedure in VA, the local judge gives the DL to the guardian in the presence of the new driver, and the guardian has the legal right to revoke it at anytime prior to the child reaching 18.