Why not just open it all the way up?
Coming soon to a Republican legislature near you:
Our laws restricting the legal age to purchase guns are contrary to accepted modes of Constitutional interpretation. When crafting Constitutional amendments, drafters have demonstrated that they know how to qualify rights based on age. See, e.g., Amendment XXVI ("The right of citizens of the United States, who are eighteen years of age or older, to vote shall not be denied or abridged by the United States or by any State on account of age." (emphasis provided)). No such age-limiting language exists in the Second Amendment. Because the rights secured by the Second Amendment are absolute ("shall not be infringed"), it follows that our children have the right to purchase and bear arms. Don't forget to like, subscribe, and vote for me!