From NRA-ILA:

Eighteen-year-olds have not been prohibited from purchasing and possessing rifles and shotguns at the federal level, and in the vast majority of states, since the founding of our country.  Nonetheless, because of the violent acts of one individual, we have seen an onslaught of legislation throughout the country that seeks to raise the minimum age to purchase and/or possess rifles and shotguns from 18 to 21.  Because common sense has taken a back seat to raw emotionalism in today’s gun control debate, some of these efforts have seen success.